TL;DR
- Gmail API is the best fit for an assistant working in an existing Google Workspace mailbox.
- Microsoft Graph is the natural choice for Outlook mail inside an existing Microsoft 365 tenant.
- Amazon SES offers inexpensive send and receive transport when your team will build storage, conversation handling and the agent-facing inbox layer.
- AgentMail is the most direct choice here for creating agent inboxes by API, with threads, drafts and inbound events already in the product.
For one existing human mailbox, use its native API. For many independent software inboxes, start with AgentMail. Use SES when owning the email-processing backend is a deliberate engineering decision. API capacity and mailbox sending limits are separate constraints.
What an agent inbox needs
An agent that handles replies needs an address, stored messages, conversation context and a way to learn that new mail arrived. It may also need drafts for review, attachments and a persistent record of what it sent.
A mailbox API gives software access to an existing mailbox. An inbox platform creates and manages inboxes as application resources. Email transport receives or sends messages, while your application supplies the inbox experience. Those are different purchases even when all three can send a reply.
Specify whether the agent works for a licensed person, represents a shared business function or needs its own independent address. An alias routes mail; it does not automatically create separate credentials, quota, stored state or sender reputation. Several agents can share an inbox if the application assigns work clearly, but separate addresses alone do not establish full isolation.
The four options compared
| Option | Identity and receiving | Conversation handling | Notification route | Buying basis |
|---|---|---|---|---|
| Gmail API | Existing Gmail/Workspace mailbox | Threads, messages, labels and drafts | Gmail watch through Cloud Pub/Sub, then history sync | Workspace user licence; standard API use currently has no additional charge |
| Microsoft Graph | Existing Exchange Online mailbox | Messages, conversation IDs, drafts and replies | Graph change notifications and delta sync | Eligible Microsoft mailbox licence plus your application |
| Amazon SES | Verified sending identity and configured receiving domain/rules | Your application stores and reconstructs conversations | Receipt actions into storage or processing services | Sending, receiving and processing meters; no conventional inbox included |
| AgentMail | API-created inbox resources | Native threads, messages, labels and drafts | Webhooks or WebSockets | Developer $20/month with 10 inboxes; paid capacity additions available |
Pricing and technical details below use official sources reviewed on 30 September 2026. The worked workloads are illustrative, not provider performance tests.
1. Gmail API
Best for an assistant inside a Google mailbox. Gmail provides thread and draft resources as well as message access. A rep can have an assistant classify replies and prepare drafts while retaining the familiar mailbox.
For new projects from 1 May 2026, Google publishes 1,200,000 quota units per minute per project and 6,000 per minute per user per project. Relevant costs are 20 units for messages.get, 2 for history.list, 10 for drafts.create and 100 for messages.send. The daily billing threshold is 80 million units per project. Google currently describes standard use as available at no additional cost, with future above-threshold billing details still to come. Existing qualifying projects retain their previous quotas. Gmail API units and pricing.
Those API units do not replace mailbox sending limits. Paid Workspace accounts have a standard 2,000-message daily sending limit, while trials have lower limits. The separate 10,000 total-recipient figure is a recipient count, not permission to send 10,000 messages. Workspace sending limits.
For new dedicated users, Business Starter's published US basis is $7/user/month on an annual plan or $8.40 on Flexible billing. An existing licensed mailbox need not add a new seat merely because an assistant accesses it. Workspace billing comparison.
Gmail watch notifications arrive through Pub/Sub and identify mailbox changes; the application retrieves the changes through history. Renew watches at least every seven days, with daily renewal recommended. Periodic reconciliation handles delayed or dropped notifications. Push documentation.
Non-fit: provisioning many independent agent identities. Mailbox licensing and user lifecycle become part of the application. Reading an existing mailbox is simple relative to building and maintaining a fleet of licensed users.
2. Microsoft Graph
Best for an existing Microsoft tenant. Graph can read messages, create reply drafts and send them. Message resources include conversation identifiers, and support change notifications and delta queries for incremental synchronisation. Message API.
Outlook's documented throttling applies to each app-ID/mailbox combination: 10,000 requests per ten minutes and four concurrent requests, with a separate upload limit. This is an API control. Exchange Online's standard 30-message-per-minute sending limit is another control, and daily recipient and tenant limits can also bind. It is inaccurate to treat 30 messages per minute as the complete Graph API throughput specification. Graph throttling and Exchange Online limits.
Microsoft's current US Business Basic base is $7/user/month paid yearly, or $5.40/user/month paid yearly for Business Basic without Teams. The email-only scenario below uses the latter, rather than adding a Teams bundle unnecessarily. Microsoft 365 plans.
Choose delegated access for work on behalf of a signed-in user, or the appropriate admin-authorised application route for a maintained service. A dedicated agent mailbox can be appropriate; Graph is not limited to borrowing a person's identity. The application should retain which agent, operation and reviewer acted, in addition to the mailbox's sender information.
Non-fit: treating Outlook as unrestricted bulk email infrastructure. It suits mailbox conversations within the tenant's operating model, not a plan to ignore Exchange sending controls because the API accepts many reads.
3. Amazon SES
Best for a team building the email backend. SES can receive mail as well as send it. Receiving needs domain routing and receipt rules in a supported region, with actions such as storing messages or invoking downstream processing. It does not provide the conventional persistent mailbox and thread resources supplied by Gmail or AgentMail. SES receiving concepts.
The current à-la-carte rates are $0.10 per 1,000 outbound emails, $0.10 per 1,000 inbound emails and $0.09 per 1,000 incoming 256 KB chunks. Outbound attachment data adds $0.12/GB. Storage, processing and other services are separate. SES pricing.
New eligible account/region combinations now default to the Essentials plan, whose first outbound tier is $0.16 per 1,000; the public pricing page allows switching to à-la-carte. Mail Manager is a separate inbound processing product with its own rates and endpoints. The calculation below deliberately selects classic SES receiving and à-la-carte sending rather than silently mixing the new default with old prices.
Your backend maps recipient addresses to agents, stores MIME and attachments, builds conversations from message identifiers, tracks bounces and provides a reviewable draft path. A catch-all receiving domain can route many addresses without buying a licence for each, but it still needs that application logic.
Non-fit: a team expecting a complete agent inbox immediately after enabling an email transport API. The low message bill is attractive precisely because it excludes much of the inbox product you must build.
4. AgentMail
Best for programmatically provisioned agent inboxes. AgentMail exposes inboxes as API resources, with messages, threads, drafts and labels. It is useful when a workflow needs an address for receiving a task, returning a reply or completing an application's email-based verification. Inbox documentation and product introduction.
The current Developer plan is $20/month, including 10 inboxes, 10,000 emails per month, 10 GB storage and 10 custom domains. Its pricing calculator lists add-ons of $2/month per inbox or domain and $2/month per additional 1,000 emails. Developer also lists 1,000 emails per day and 100 per five minutes; monthly capacity does not remove burst controls. Free has three inboxes and 3,000 monthly emails; Startup is $200/month with 150 inboxes, while Enterprise has custom terms. AgentMail pricing.
Webhooks and WebSockets provide inbound events. Retain a durable message record before triggering the agent so duplicate delivery cannot launch two independent replies. Webhook overview.
AgentMail also documents client_id for idempotent resource creation and an Idempotency-Key header for sends. Send keys expire 24 hours after completion, so your own longer-lived action ledger still matters. Idempotency documentation.
Non-fit: buying it merely to let an assistant read an existing Microsoft or Google mailbox. Use the mailbox's native API for that job. AgentMail's value is the new software inbox and its surrounding primitives.
A received-email workflow
Suppose an authorised customer emails an agent-owned service address with an account-research request. The application should:
- Receive the notification and save the provider message ID, inbox ID, sender and content reference.
- Deduplicate that message before creating a job.
- Retrieve the conversation and relevant attachments, then produce a draft answer.
- Route the draft through the workflow's review rule and record the decision.
- Send the approved response once and retain the returned message ID.
Gmail implements the first steps through watch/history and mailbox reads. Graph uses notifications and message/delta retrieval. SES supplies the received message to your processing path, which must reconstruct the thread. AgentMail supplies the inbox, thread and received-event route directly.
An incoming email is input data; it should not grant itself permission to alter CRM ownership, send unrelated messages or disclose another customer's files. Keep the agent's actions tied to the request and the configured workflow. The signal-based outbound playbook explains how to use the resulting research in a sales process.
The cost of 50 agent identities
Assume 50 independent inbox identities, 2,000 outgoing single-recipient messages and 2,000 incoming messages monthly. Each incoming message fits in one 256 KB chunk, and outbound messages have no attachments. This is a low-volume reply workflow, not a bulk campaign.
| Option | Identity/transport calculation | Supported base cost |
|---|---|---|
| Gmail with 50 new Business Starter users | 50 × $7 annual-plan monthly equivalent | $350/month equivalent; Flexible billing is 50 × $8.40 = $420/month |
| Graph with 50 new Business Basic no-Teams users | 50 × $5.40 annual-plan monthly equivalent | $270/month equivalent |
| SES, selected à-la-carte transport | 2 × $0.10 outgoing + 2 × $0.10 incoming + 2 × $0.09 incoming chunks | $0.58 transport, plus your inbox backend and storage |
| AgentMail Developer with inbox additions | $20 + 40 additional inboxes × $2 | $100/month before additional storage or domains |
The AgentMail workload remains below the published 10,000-email monthly headline even when both directions are counted together. Spread the work within its daily and five-minute controls. The 50 identities do not each require a separate domain; use only the domains the workflow actually needs.
For an existing Gmail or Microsoft mailbox, the incremental licence cost may be zero. Those rows deliberately price 50 new licensed users, not aliases or shared-mailbox arrangements. Shared routing can reduce identity costs if separate licensed mailboxes are unnecessary, but it changes the isolation model.
For Gmail, an illustrative one-pass processing pattern, 2,000 message reads, 2,000 history calls, 2,000 draft creations and 2,000 sends, uses 40,000 + 4,000 + 20,000 + 200,000 = 264,000 quota units over the month, before additional calls and watch renewals. The method units describe API work; they are not a dollar charge per message, and the application's peak rate still matters.
Now include operating effort. Assume $20 monthly model usage for every option. For the SES backend, budget $25 hosting/storage and four maintenance hours at $75/hour: $0.58 + $25 + $300 + $20 = $345.58/month. For AgentMail, assume one maintenance hour at the same rate: $100 + $75 + $20 = $195/month. That illustrative $150.58 difference outweighs SES's cheaper message transport.
If building the SES inbox layer takes an additional 40 engineering hours at $75/hour, setup adds $3,000. It must save more than $250 monthly over a year to repay that build cost before maintenance. SES can be the right choice at scale or with an existing backend; a sub-dollar transport bill alone is not that case. These labour figures are assumptions, not estimates supplied by either vendor.
Identity, recovery and delivery
Record the agent's identity and operation independently of the mail provider. A dedicated sender address makes the role clear, while the application ledger records who prepared and approved the response. Gmail and Graph can use dedicated business mailboxes; AgentMail can provision inboxes as resources; SES can route addresses into your own identities.
Persist thread references and send outcomes. If a response times out, reconcile with the provider before blindly resending. A draft failure can be retried without resending an already completed reply. A repeated inbound event should locate the existing job.
Authenticate the sending domain and handle bounces and suppressions. Distinct inboxes do not guarantee distinct reputation when they share a domain or transport infrastructure. An API-accepted send also does not prove inbox placement or that the recipient read it. Base delivery decisions on actual feedback rather than a claimed formatting trick or automatic exclusion of enterprise mail gateways.
Which option to choose
For a small assistant working in a human's established mailbox, use Gmail API or Graph, according to the team's existing platform. Preserve the mailbox experience and implement an explicit draft-to-send workflow.
For new agent identities created as part of an application, choose AgentMail first. Its inbox, thread and event primitives remove a substantial amount of backend work, and its published Developer additions make a modest fleet budgetable.
Choose SES when your team already owns the email-processing application or has a clear reason to build it. Price the selected SES plan, receipt processing, storage and maintenance together. Transport economics become compelling when that engineering layer is already justified.
FAQ
Is Gmail API free?
Standard API use currently has no additional charge under Google's documented model. The Workspace mailbox, application infrastructure and models remain separate. Google has announced future above-threshold billing rather than publishing a new per-message price here.
Does Graph's request capacity let an agent send unlimited email?
No. Outlook API throttling and Exchange sending controls both apply. Read-heavy mailbox work and high-volume outbound sending are different workloads.
Does SES give each agent a stored inbox?
It supplies transport and receipt processing. Your application builds message storage, agent routing and conversation handling. That distinction explains the very low transport cost.
Should each agent have its own address?
Use separate addresses when identity and routing need to be distinct. Separate credentials, data access and reputation depend on the surrounding design; an address or alias by itself does not provide all of them.
