Resend is an excellent transactional email API. MCPmailer is a mailbox for your agent, with threading, waiting, and human oversight.
Transactional email APIs · Resend claims checked July 2026
Resend is the modern transactional sender: a clean API, React Email for templates, sensible deliverability defaults, an SMTP relay, and inbound handling on every plan including free. If your app sends password resets and receipts, it is a fine default and we recommend it for that.
For anything your product sends to a person, use Resend or something like it. Templated one-way mail, batch sends, open and click tracking: that is its job and we deliberately do none of it. We have no templating engine of our own, no campaigns, and a hard cap of five recipients per message.
A transactional API answers "deliver this message". An agent needs answers to "what came back, which conversation was it part of, and what do I do while I wait". Resend can hand you inbound mail as a webhook; thread matching, storage, search, per-agent identity, and the waiting are then yours to build. Those are the product.
Resend inbound posts you a parsed message. There is no mailbox object that owns it, so there is nothing to list, nothing to search, and no thread to continue. Everything an agent needs in order to answer sensibly, the earlier messages, what was promised, whether this person already complained, is state you have to keep.
A transactional plan gives you one number per month. Our pre-send pipeline classifies every message: replying into a thread the recipient started is near-unrestricted, while first contact with a stranger is quota gated and can be refused with a reason the agent understands. That is the control you actually want when a model is choosing the recipients.
A Resend API key is scoped to a domain and can send as anything on it. Ours is scoped to one agent, and that agent cannot see other identities, notes, or vault secrets in the workspace unless granted. Ten agents means ten revocable keys and ten separate audit trails.
Every row is a claim we could defend with their documentation open beside it. Where they are ahead, the row says so.
Most customers do not switch. They split the traffic by what the mail is for, which also keeps the two reputations apart.
Leave product mail on Resend
Receipts, resets, and notifications keep their templates, their tracking, and their deliverability history.
Give the agent its own subdomain
agents.yourcompany.com, separate from the domain your product sends from, so agent behaviour can never affect password-reset delivery.
Retire the inbound webhook and the messages table
The handler, the thread matcher, and the store you built around Resend inbound all collapse into read_message, get_thread, and search_inbox.
Point the agent at MCP
One server entry with a scoped key. If the agent is not an MCP client, /v1 takes the same key.
Yes, and plenty of people start there. You will then write thread matching, a message store, a search index, a wake-up mechanism, and per-agent identity. That collection of things is what MCPmailer is, so the question is whether you want to own it.
Not for product email. Keep it for receipts and resets. We are for the addresses your agents converse from, and keeping them on separate subdomains is good practice anyway.
They are nearly identical in volume: 3,000 emails a month at 100 a day. Resend gives you one domain; we give you three agent inboxes on an instant subdomain, plus inbound storage, threading, and search on the free tier.
For cold, templated, one-way mail their tooling and history are strong. Most agent mail is a reply inside a thread the recipient started, which is the easiest mail there is to deliver, so the deciding factor is usually the conversation model rather than the wire.
Keep your transactional sender for the mail your product sends. This is for the addresses your agents converse from, which is a different job with different failure modes.
The free tier is 3,000 emails a month across three agent inboxes, no card, with receiving, threading, and search included. Enough to watch a real conversation happen before you decide.
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