TL;DR
- Mailforge is the shared-infrastructure option when fast domain and mailbox provisioning matters and the agency accepts a shared IP pool. Its slot model charges for capacity, including unused slots.
- Infraforge is the dedicated-server alternative when the team wants more infrastructure control and is willing to manage that responsibility. A dedicated IP is a separately priced option, not something to assume from “private.”
- Maildoso offers packaged SMTP mailbox capacity. Its public 30-mailbox and 300-mailbox plans create an awkward 50-mailbox buying decision; price a custom package or the next bundle, and check domain transfer terms.
- InboxKit currently lists per-mailbox managed Google Workspace or Microsoft 365 accounts, with admin-panel access and no slot minimum. Verify the exact tenant and domain arrangement for each client.
- Direct Google Workspace or Microsoft 365 gives the clearest route to client-owned administration, but the agency must handle licensing, DNS, security, support and policy compliance itself.
- At 50 mailboxes, the winning number is not the cheapest mailbox. It is the cost of usable, governed mailboxes that can be paused, recovered and handed back to the right client.
Quick comparison
Five storefronts can produce five very different operational contracts. Before comparing a price, ask who controls the domain, tenant, DNS, mailbox credentials, sending IP, reply archive and transfer on termination. A managed mailbox can be quick to provision yet costly to untangle if it belongs to the agency while the client believes it owns the sending identity.
| Option | Operating model | Published entry unit | Question that decides the purchase |
|---|---|---|---|
| Mailforge | Shared IP pool; domain and mailbox slots | Calculator shows $3/mailbox/month at selected state, billed monthly | Can each client export domains and mailboxes cleanly? |
| Infraforge | Dedicated servers; optional IP add-on | Calculator shows $4/mailbox/month, billed quarterly; IP add-on $99/month | Which server and IP are assigned to each client? |
| Maildoso | Packaged SMTP mailbox licences | 30 for $75/month; 300 for $225/month | What is the exact 50-mailbox package and transfer process? |
| InboxKit | Managed Google or Microsoft mailboxes | $2.50/mailbox/month annually; $3 monthly | Who owns the admin/tenant and what extra tenant cost applies? |
| Direct Google or Microsoft | Client-controlled business tenants | Both have $7/user/month annual entry plans | Can the client or agency administer 50 identities safely? |
The unit matters. A slot is billable capacity even when no address exists. A bundled mailbox licence may be cheaper per unit but require buying 300 to operate 50. An annual user licence can remain payable after a campaign ends. A per-mailbox managed service can have separate domain, tenant, warming or monitoring charges. None of those rates includes the sequencer, list research, email verification or the people who investigate an incident.
What changes at 50 mailboxes
Imagine an agency with five clients, ten mailboxes per client. One client pauses its programme, another has a reply routing failure, and a third asks to take its domains and mailboxes in-house. If the agency's records consist of a spreadsheet of passwords, it cannot reliably answer which domain, tenant, sequencer campaign and suppression list belong to each client. Price only becomes meaningful after this problem is solved.
Keep a registry with client ID, domain registrar and owner, DNS administrator, mailbox address, provider, underlying tenant or server/IP where known, campaign assignment, reply destination, authentication state, start date, renewal date, health state and offboarding method. A password should not be stored in that registry; record the credential owner and vault reference instead. The registry is not bureaucracy. It lets an operator stop all sends from an affected asset without touching unrelated clients.
The blast radius also changes. Fifty mailboxes spread across five genuinely separate clients may be safer to govern than fifty pooled under one tenant or provider account. Conversely, buying from five vendors does not automatically create provider diversity: two storefronts can sit on similar underlying infrastructure. Ask for the actual server, tenant and IP model. When a vendor says “dedicated,” determine whether that refers to a server, tenant, IP, domain, or only a workspace in its dashboard.
Make client ownership explicit in the service agreement. A client-owned domain with delegated DNS access is easier to hand over than a domain registered in the agency's name and subject to an undocumented transfer process. If the agency owns infrastructure for operational reasons, document the transfer right and run a test before the relationship ends. Preserve replies, suppression and opt-out history during any move.
1. Mailforge: shared infrastructure and fast provisioning
Mailforge's pricing page describes a shared IP pool, automated DNS setup and mailbox slots. Slots allow an operator to delete and recreate mailboxes without purchasing new capacity; a slot still costs money while unused. The site says the minimum purchase is ten slots. Its displayed calculator state shows $3 per mailbox per month on monthly billing for 25 selected slots and $14 per year for a .com domain. Treat those as current calculator examples and reconfirm the 50-slot checkout total.
Why an agency would choose it. The team needs to set up many low-volume sending identities for separate campaigns without hand-configuring every SPF, DKIM and DMARC record. Automated setup and a slot-based replacement workflow may lower operations time. Mailforge also says users can edit DNS and manage domains. In a pilot, verify what that means for a client-owned registrar and whether the client can retain the configuration outside the vendor dashboard.
The real tradeoff. Mailforge itself describes the infrastructure as shared. The agency does not receive a private sending environment by buying another mailbox slot. Shared infrastructure can be economical, but it creates an external dependency when other users or an underlying provider affect reputation or service. Ask for incident communication, IP visibility, reply export and the procedure to stop one client without impacting others. Do not accept vendor testimonials as evidence that your campaigns will reach inboxes.
Buying check. Request a line-item 50-slot and 100-slot quote with domains, billing term, renewal, DNS control and replacement rules. Test the exact sequencer connection you use. Then remove a mailbox and domain from a sandbox client and confirm what the client receives at handoff. If the agency needs direct Google or Microsoft accounts, Mailforge's shared model is a different product; price those alternatives separately.
2. Infraforge: dedicated-server infrastructure
Infraforge describes private email infrastructure using dedicated servers. Its displayed calculator state shows $4 per mailbox per month with quarterly billing for 25 selected slots, a ten-slot minimum and automated DNS setup. It also lists an IP address add-on at $99 per IP per month, billed quarterly. That separate line matters: a dedicated server claim does not prove a dedicated IP is included in the base mailbox price.
Why an agency would choose it. A team may want to separate a large client from a pooled sending environment and understand more clearly which infrastructure serves it. If a client has enough mailboxes to justify its own workspace or server arrangement, Infraforge can be a candidate. The product lists API access and an optional Masterbox view, which may matter when operators need to provision or monitor many addresses.
The real tradeoff. Greater control also concentrates responsibility. A private server does not forgive poor list quality, a broken DKIM record or sudden volume changes. Nor is one dedicated IP necessarily better than a well-managed shared environment: its reputation must be earned and maintained by the sender's actual behaviour. Ask what the service monitors and what incident recovery the vendor performs versus what the agency must do.
Buying check. Map server, tenant and IP boundaries for each client. Request the 50-slot total, number of IPs included, optional IP charges, minimum term, replacement rights and client export. Test a disabled mailbox and a DNS change with support present. If a client must leave, establish whether the domain, mailbox contents and reply history can move without losing live conversations. A $4 slot is not the full dedicated-infrastructure cost when an IP and operational labour are needed.
3. Maildoso: bundled SMTP capacity
Maildoso sells outbound SMTP mailbox packages and describes automated authentication, sequencer connections and an API. Its current FAQ lists 30 mailboxes for $75 per month, 300 for $225 per month, and 1,000 for $499 per month. The often-advertised $0.49 per mailbox is therefore the 1,000-mailbox tier, not the cost of an agency's first 50. The provider says nonstandard volumes can be purchased through its package/add-on path; ask for the exact 50-mailbox price rather than extrapolating.
Why an agency would choose it. A team planning several hundred similar outbound addresses may value a bundle, central provisioning and API-based creation. Maildoso says purchased domains are registered with it while the service runs and can be returned at the end of the subscription. That is a concrete ownership term to examine. It also allows externally owned domains to be connected, which may better suit a client-owned setup.
The real tradeoff. The public 300-mailbox bundle can look cheap at $0.75 per slot, but paying for 300 when only 50 are active means $225 monthly, or $4.50 per active mailbox before domains and labour. The economic picture changes if the agency soon fills the bundle, but capacity without client demand is not savings. The FAQ also describes provider-controlled infrastructure and automated recovery; an agency needs to know when a mailbox was paused or moved and whether its own campaign system stopped sending correctly.
Buying check. Ask whether a 50-mailbox custom package has the same API, replacement, monitoring and domain rights as the standard plans. Test one domain transfer in both directions, including DNS and reply continuity. Maildoso's page states a $12 yearly domain registration fee and a planned rise to $15 on 1 October 2026; verify the price on the purchase date. Do not substitute the vendor's advertised deliverability percentage for a controlled test on your client mix.
4. InboxKit: managed Google and Microsoft mailboxes
InboxKit's current pricing page lists managed Google Workspace and Microsoft 365 mailboxes at $2.50 per mailbox per month on annual billing or $3 monthly, with no plan or slot minimum. It lists admin-panel access, monitoring, workspaces, integrations and API access with each mailbox. An Azure tenant is $30 per tenant per month for up to 100 mailboxes where that option applies. Warmup is separately listed at $3 per mailbox per month. Those add-ons should be chosen for a defined need, not silently bundled into a “mailbox price.”
Why an agency would choose it. Some clients want the operational convenience of a managed provider while requiring familiar Google or Microsoft mailbox types. A per-mailbox model fits uneven client growth better than a large minimum bundle. The ability to use client domains and see an admin panel may support a cleaner handoff than credentials alone, but the exact admin role and underlying tenant ownership must be checked.
The real tradeoff. “Google mailbox” or “Microsoft mailbox” does not by itself tell the client who controls the tenant, billing relationship, security policy, IP assignment or transfer rights. For Microsoft, an Azure tenant add-on can materially change a 50-mailbox comparison. Ask whether each client receives a separate tenant, whether the agency can delegate administration, and what data and settings can leave the platform. Also confirm whether the public annual rate is billed up front or from wallet renewals as described on the page.
Buying check. Provision two client test domains, connect the real sequencer and verify MFA, SPF/DKIM/DMARC, replies, alerts and admin access. Then simulate a client exit. Can the client independently administer the domain and mailbox, or must it migrate to new accounts? Record that answer beside the price. Older InboxKit search snippets list plan bundles; the live pricing page currently uses per-mailbox billing, so use the current checkout rather than an old comparison post.
5. Direct Google Workspace or Microsoft 365
Buying from Google or Microsoft directly is the clearest path when the client wants the tenant, billing and administrative controls in its own account. Google Workspace Business Starter is $7 per user per month with an annual commitment or $8.40 on its flexible plan in the published US comparison. Microsoft 365 Business Basic is $7 per user per month paid yearly in the US page. Regional rates, tax, contract and reseller terms can differ.
Why an agency would choose it. A high-value client wants durable ownership, existing security policies, managed users, straightforward offboarding and standard support. The agency can receive delegated access instead of holding the whole tenant. For a small number of important senders, the higher licence price may be easier to justify than uncertainty over identity ownership.
The real tradeoff. The agency or client must create and administer users, verify domains, configure DNS, connect the sequencer, monitor abuse signals and respond to account restrictions. Ordinary business mailboxes are not unlimited campaign infrastructure. Read the provider's current Google sender guidelines or Microsoft authentication guidance, plus applicable service terms, before designing a send policy. A published technical limit is not a recommended cold-email volume.
Buying check. Create the tenants in the client's name where appropriate; document delegated admin, billing, MFA and recovery. Test that the client can revoke agency access while keeping its mail and DNS. Count the administrator's hours in the cost model. A direct licence is not automatically the best choice for every temporary or low-value campaign, but it is the cleanest benchmark for ownership.
A 50-mailbox cost model
Use five clients × ten mailboxes, with 20 domains across those clients, as a sample. These are comparison inputs, not a recommendation to put a fixed number of mailboxes on every domain. The exact domain count should follow brand, provider and programme requirements. Assume US public prices, no taxes, no sequencer, no paid monitoring and no labour in the first pass.
| Option | Illustrative hosting maths | Important exclusion |
|---|---|---|
| Mailforge | 50 × $3 = $150/month at displayed monthly slot rate; 20 .com domains × $14 = $280/year |
Confirm 50-slot calculator rate, domain ownership and support scope |
| Infraforge | 50 × $4 = $200/month at displayed quarterly slot rate; one optional IP adds $99/month | Confirm whether an IP is needed and how clients are isolated |
| Maildoso | Standard 300-mailbox package = $225/month for 50 active; custom 50 quote may differ | Domains and underused capacity; 30-mailbox plan insufficient |
| InboxKit | 50 × $2.50 = $125/month equivalent on annual billing, or 50 × $3 = $150/month monthly | Azure tenant, domains and optional warmup are additional where relevant |
| Direct Google or Microsoft | 50 × $7 = $350/month equivalent with annual commitment | Admin labour, domain costs, sequencer and contract terms |
This table compares unlike models intentionally so the omissions are visible. Mailforge and Infraforge calculator pages show rates in selected states, so get an exact 50-slot total. InboxKit's annual rate has a different cash commitment from a monthly plan. Maildoso's 300-mailbox package offers unused expansion capacity. Direct licences include broader workplace services, but the agency may not use them. The second pass should add domains, tenant/IP options, DNS work, monitoring, support and a likely replacement rate for each.
For a decision-quality estimate, calculate annual cost per usable client-owned mailbox-month: total year-one charges plus operating labour, divided by the number of mailbox-months that stayed authenticated, connected, monitored and correctly assigned to a client. If a cheap package loses reply continuity or takes days to offboard, its effective cost rises. Do not value a mailbox simply because it exists in a dashboard. The adjacent cold email infrastructure providers comparison covers the broader supplier landscape.
Pilot ownership before scaling
Pilot with two client-like environments and ten to twenty mailboxes total. Do not start with the easiest internal domain only. Use one domain registered outside the vendor, one vendor-purchased domain if that is the proposed setup, and the actual sequencer. Record DNS records before setup and after. Confirm which user can change them and how long a change takes to propagate.
Send controlled internal messages and verify that SPF, DKIM and DMARC are configured and aligned for the sending identities. Then check reply routing, campaign stop controls and suppression propagation. A provider's automated DNS setup can save time; it does not absolve the agency from verifying the result. Run a seed test if placement diagnosis matters, but do not call its percentage the actual inbox rate of the campaign. Our inbox-placement testing guide explains that distinction.
Simulate three operational events: one mailbox is restricted, one client asks to pause immediately, and one client terminates. The team should stop new sends from the affected identity, keep existing replies accessible, preserve unsubscribes, rotate or revoke credentials, and export what the contract promises. Time each step. A provider that provisions 50 mailboxes in minutes but takes weeks to return a domain fails the agency's continuity requirement.
Write the pass criteria before purchase: 100% of mailboxes assigned to the correct client and campaign; no unidentified domains or accounts; all required authentication checks pass; a same-day stop can be executed; and the client can receive its domain, data and access in the agreed form. Document any exceptions and owner. That gives the agency a real operational reason to prefer one vendor over another.
How to decide at 100 and 500 mailboxes
At 100, compare the marginal cost of adding 50 with the cost of a second provider or tenant. The agency may have enough volume to negotiate a better unit price, but it also has more to lose from a shared outage. Split clients according to actual infrastructure boundaries, not just dashboard folders. Make renewal and capacity forecasts per client so unused annual licences do not accumulate.
At 500, procurement becomes an operations design. Ask for an exportable inventory, API or bulk controls, audit logs, role-based client access, dedicated support, incident notices, clear data-processing terms, and a rehearsed exit. Maildoso's 1,000-mailbox $499 tier or another bulk quote may look dramatically cheaper than 500 direct licences. That arithmetic is relevant only if the infrastructure, domain rights and support model satisfy the clients' needs. A low unit price is not compensation for an unbounded cross-client failure.
Keep the sender stack separate from the infrastructure decision. A mailbox host does not necessarily include campaign sequences, verified prospect data, replies in the CRM or deliverability investigation. Budget each stage and appoint an owner. If a programme is underperforming, determine whether the cause is targeting, messaging, list hygiene, authentication or provider treatment before paying for more identities.
Use the Cold Email Deliverability Checklist to inspect authentication and sending readiness across the mailbox fleet before launching campaigns.
FAQ
How many domains do 50 cold email mailboxes need?
There is no universal ratio. Provider policies, brand architecture, client separation and the number of genuinely distinct sending identities matter. Start with client ownership and failure boundaries, then test the planned domain-to-mailbox arrangement against the provider's actual terms. Do not use a vendor's suggested daily volume as a guarantee of safe sending.
Does private infrastructure guarantee better inbox placement?
No. A dedicated server or IP changes who shares a failure domain, but it does not fix authentication, bad lists, complaints or weak targeting. Test actual provider-level results and production signals before treating infrastructure type as the explanation.
Should the client or agency own outbound domains?
For durable client programmes, client ownership usually makes continuity and offboarding clearer. The agency can be delegated DNS or admin rights. If the provider or agency registers a domain, put the transfer right, process and timing in writing and test it before scale.
What is the cheapest option at exactly 50 mailboxes?
The public figures above do not identify a universal cheapest configuration. InboxKit's annual 50-mailbox arithmetic is $125/month equivalent before any tenant option; Mailforge's selected rate is $150/month before domains; Maildoso's public 300-mailbox package is $225/month, but a custom 50 quote may differ. Billing term, domains, tenant/IP charges and labour can change the ordering.
Can I use a managed provider's Google or Microsoft mailbox without a client tenant?
The product label alone does not answer who owns or controls the tenant. Ask for the exact admin role, billing account, domain registration, data export and transfer process. Test those controls with a pilot client, then document them in the client agreement.
What should be tested before moving from 50 to 500 mailboxes?
Reconcile the registry, verify DNS and authentication, pause sends across one client, recover a restricted mailbox, export replies and suppression state, and complete a domain handoff. Then test bulk provisioning and audit logs. Expansion is justified only after these controls work at the smaller scale.
