The right email API for an agent depends less on price than on one question: does your agent only send, or does it need to hear back? Sort the field by that and the choice gets short.
If your agent only sends
Reports, alerts, summaries to the human who asked for them. Here any good transactional API will do, and the deciding factors are developer experience and volume pricing. Resend is the developer favourite: a clean API, React Email templates, and a free plan with 3,000 emails a month (100 a day). Pro is $20 for 50,000. Postmark has the strongest reputation for inbox placement on transactional mail, from $15 a month for 10,000. SendGrid and Mailgun are the long-standing high-volume options; SendGrid Essentials starts at $19.95, Mailgun Basic at $15 for 10,000.
If you are only sending, you do not need a mailbox, and an agent-specific product is more than you need.
If your agent holds a conversation
Support, scheduling, chasing invoices, signing up for services and reading the verification code. Now the agent needs an address people reply to, the reply delivered somewhere, and the thread kept together. This is where the field splits into two designs.
The first design is inbound parsing on a sending API. Resend, Postmark (Pro plan, $16.50), SendGrid and Mailgun all accept mail for your domain and post it to a webhook. Resend's webhook carries metadata and you fetch the body through its Receiving API. You store messages and stitch threads yourself. It suits teams that already keep conversations in their own database.
The second design is a hosted inbox the agent owns. AgentMail is built for agents first: create an inbox with one call, receive by webhook or websocket, and it has a generous free tier (3 inboxes, 3,000 emails a month). Custom domains start at $20 a month for 10 inboxes, and 150 inboxes are $200. Faivelo gives the agent a real mailbox on your domain, driven over REST or a hosted MCP server, that a person can also open in webmail or on a phone. Mailboxes are unlimited at a flat price, so the twentieth agent inbox costs the same as the first. Growth is $12 a month with MCP included; the message.received webhook comes with Pro at $24.
An agent that cannot see the reply has to be told what happened. One that can see it just knows.
If your agent works inside someone's existing inbox
An assistant that triages your Gmail or drafts in your Outlook needs to connect to the account you already have. Nylas is the established API for that, with 5 connected accounts free and paid plans from $15 a month; it has also added hosted Agent Accounts, 3 free and 10 on Essentials. Gmail and Microsoft Graph directly are free but involve OAuth review. Faivelo's MCP server does this for mailboxes hosted on Faivelo only, not for Gmail.
Side by side
| Service | Agent gets | Receives replies | Vendor MCP server | Entry price |
|---|---|---|---|---|
| Faivelo | Real mailbox on your domain, webmail and IMAP too | Yes, mailbox plus webhook (Pro) | Hosted, OAuth or API key | $12/mo flat, unlimited mailboxes |
| AgentMail | API inbox, built for agents | Yes, webhook or websocket | Official | Free (3 inboxes), $20 for custom domains |
| Resend | Sending API | Inbound webhook, fetch body by API | Hosted | Free (3,000/mo), $20 Pro |
| Postmark | Sending API | Inbound webhook on Pro | Official, experimental, runs locally | $15/mo, inbound from $16.50 |
| SendGrid | Sending API | Inbound Parse webhook | None official | $19.95/mo after a 60-day trial |
| Mailgun | Sending API | Inbound Routes (1 free, unlimited from $35) | Official, runs locally | Free 100/day, $15 Basic |
| Nylas | Connects existing Gmail/Outlook; hosted Agent Accounts | Yes | Official | Free (5 accounts), $15 Essentials |
Prices from each vendor's pricing page, checked 4 October 2026. Vendor MCP server means one the company itself publishes; community wrappers exist for all of them.
How to choose
Sending only: Resend or Postmark. Throwaway inboxes created by the hundred, no domain needed: AgentMail. Agents that should look and act like staff on your own domain, where a person can read along and the price does not climb with every new inbox: Faivelo. Acting inside a person's existing Gmail: Nylas or the provider's own API.
Try the mailbox route
Add your domain, create an agent key and a mailbox, and connect Claude or your own code. Fourteen days free, no card, and MCP is switched on during the trial.