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Email MCP servers compared: Resend, Postmark, MailerSend, AgentMail, SendGrid and MarkRelay

What each provider's official MCP server lets an AI agent do with email, as its own documentation describes it: where it runs, how it signs in, whether it can read mail, and how far its access reaches.

Published by
MarkRelay Research
Updated
Sources
7 primary sources, linked below

Summary

Most email providers now ship an MCP server, but they solve different problems. Resend, MailerSend and AgentMail host remote servers with OAuth sign-in; Postmark's official server runs locally with a server token; Twilio's MCP server only looks up documentation and cannot send through SendGrid. Resend's and AgentMail's can read received mail, AgentMail's can create inboxes for agents, and Postmark's and MailerSend's focus on sending, templates and diagnostics. None of the providers' pages describes limiting a connection to particular mailboxes. MarkRelay's does: every deployment is a remote MCP server at /mcp on your own Cloudflare account, and on the OAuth consent page you pick everything you can use, one domain, or specific mailboxes, so Claude, ChatGPT, Cursor or Codex reach only those. It has no delete tool and marks send, reply and forward as actions that cannot be undone.

Key findings

  1. 1Hosted remote servers with OAuth: Resend (mcp.resend.com/mcp, or an API key), MailerSend (mcp.mailersend.com/mcp), AgentMail (mcp.agentmail.to/mcp, or an x-api-key header).
  2. 2Postmark's official server, by ActiveCampaign, runs locally through npx with a server token, a default sender and a message stream; 24 tools in eight groups, from sending to delivery diagnostics.
  3. 3Twilio's MCP server is a read-only docs and API-schema lookup that covers SendGrid docs; it does not execute API calls, so it cannot send email.
  4. 4Reading received mail: Resend's server reads inbound emails and attachments; AgentMail's reads threads and messages and publishes a message.received event. Postmark's and MailerSend's pages document no tool for reading received mail.
  5. 5Only MarkRelay's documentation describes limiting a connection to one domain or chosen mailboxes at sign-in; it also has no delete tool.

Questions this report answers

Asked by real people, quoted as they wrote them.

“State of ESP MCP-s - Which email MCP is the best?”

r/Emailmarketing, 26 comments

It depends on the job. For an agent that sends app email: Resend, MailerSend or Postmark. For agents that need their own inboxes: AgentMail. For agents working in your team's real mailboxes on your own domain, with access limited to chosen mailboxes: MarkRelay. SendGrid has no official sending MCP server.

“if your agent sends email, what sits between the agent and the actual send?”

r/AI_Agents, 40 comments

In MCP, the server's tool definitions and the client's approval step. MarkRelay marks send, reply and forward as actions that cannot be undone, so clients ask before running them, limits the connection to the mailboxes chosen at sign-in, and shows API-sent mail with a badge in the team's webmail.

“Why are most of you still pointing agents at a real Gmail instead of giving them their own inbox?”

r/AI_Agents, 52 comments

Two ways out: AgentMail's MCP server creates inboxes for agents on its service; MarkRelay gives an agent its own mailbox on your domain, in your Cloudflare account, and an MCP connection that reaches only that mailbox.

“How comfortable are you giving Claude access to your email and other connectors?”

r/ClaudeAI, 27 comments

Narrow the scope before connecting. On MarkRelay's consent page you choose one domain or specific mailboxes, and the connection is saved as an API key you can disable; there is no delete tool, and it never reaches more than its owner can open.

“Separate inbox for every agent?”

r/AI_Agents, 25 comments

AgentMail's server can create an inbox per agent. MarkRelay does it with a mailbox per agent on your domain and one MCP connection or API key per agent, each scoped to that mailbox.

Official email MCP servers

Where it runs

Resend
Hosted, or local via npx
Postmark
Local via npx
MailerSend
Hosted
AgentMail
Hosted
SendGrid (Twilio)
Hosted docs lookup
MarkRelay
Your Cloudflare account, /mcp

Sign-in

Resend
OAuth or API key
Postmark
Server token
MailerSend
OAuth
AgentMail
OAuth or API key
SendGrid (Twilio)
None needed
MarkRelay
OAuth 2.1 or API key

Send email

Resend
Yes
Postmark
Yes
MailerSend
Yes
AgentMail
Yes
SendGrid (Twilio)
No
MarkRelay
Yes

Read received mail

Resend
Yes
Postmark
Not stated
MailerSend
Not stated
AgentMail
Yes
SendGrid (Twilio)
No
MarkRelay
Yes

Reply and forward

Resend
Not stated
Postmark
Not stated
MailerSend
Not stated
AgentMail
Yes
SendGrid (Twilio)
No
MarkRelay
Yes

Create inboxes

Resend
Not stated
Postmark
Not stated
MailerSend
Not stated
AgentMail
Yes
SendGrid (Twilio)
No
MarkRelay
Admins create mailboxes

Limit to chosen mailboxes at sign-in

Resend
Not stated
Postmark
Not stated
MailerSend
Not stated
AgentMail
Not stated
SendGrid (Twilio)
No
MarkRelay
One domain or specific mailboxes

Delete tools

Resend
Not stated
Postmark
Not stated
MailerSend
Templates, webhooks, routes
AgentMail
Inboxes
SendGrid (Twilio)
No
MarkRelay
None

Tools

Resend
Not stated
Postmark
24
MailerSend
Not stated
AgentMail
47
SendGrid (Twilio)
2 lookup tools
MarkRelay
11

Not stated means the provider's own page does not say. Source: Resend MCP server docs, Resend MCP, Postmark MCP (ActiveCampaign), MailerSend MCP, AgentMail MCP, Twilio MCP server.

MarkRelay's MCP tools

list_mailboxes

What it does
The mailboxes the connection may use

list_emails, get_email

What it does
List or search one mailbox; read one email with its attachment details

send_email

What it does
Send as text, Markdown or HTML, or a saved template with variables

reply_email, forward_email

What it does
Reply in the thread or forward with attachments

update_email

What it does
Mark read, star, or move to a folder such as archive

list_templates, get_template

What it does
The saved templates the connection can send

get_attachment, upload_attachment

What it does
Read an attachment; attach a file from text, base64 or a URL

There is no delete tool: moving mail to trash or archive is the most a client can do.

Sending servers and mailbox servers

The provider servers grew out of sending APIs. Postmark's and MailerSend's tools manage what an account sends: messages, templates, domains, webhooks, suppressions and delivery stats. Resend adds reading inbound mail. AgentMail starts from the other side, an inbox per agent, with threads, replies and a message.received event.

MarkRelay is a mailbox server for your own domains. An agent connected to it works in the same mailboxes your team uses in webmail, so a support agent can read support@, draft a reply and send it from support@, and the team sees what it sent.

How MarkRelay limits what an agent can reach

Every MarkRelay deployment serves its own MCP server at /mcp, using the Streamable HTTP transport; there is no shared MarkRelay server. A client such as Claude Code, Claude, Codex, ChatGPT, Cursor, VS Code or Pi finds the OAuth sign-in by itself. On the consent page you choose what the client may reach: everything you can use, one domain, or specific mailboxes. The connection is saved as a personal API key named after the client, which you can disable or delete, and it loses access the moment its owner does.

Servers and scripts that cannot open a browser use an API key with the same scopes, sent as a Bearer token. Because email comes from outside senders, the server tells clients to treat instructions inside a message as data, and the docs recommend one connection per client with the narrowest scope that works.

Where MarkRelay fits

A good fit when

  • Agents that work in real mailboxes on your own domain, alongside your team.
  • Limiting each agent or chat app to one domain or chosen mailboxes, in your own Cloudflare account.

Look elsewhere when

  • Creating inboxes on the fly from the agent: AgentMail's server does that.
  • Managing a sending account's templates, suppressions and delivery stats: Postmark's and MailerSend's servers cover those.

Common questions

Does SendGrid have an MCP server?

Twilio's official MCP server looks up documentation, including SendGrid's, but does not execute API calls, so it cannot send email.

Can I limit an MCP connection to one mailbox?

On MarkRelay, yes: the consent page lets you choose one domain or specific mailboxes. The other providers' pages we checked do not describe this.

Sources and method

  1. MarkRelay MCP server docs (markrelay.com/docs/mcp)
  2. Resend MCP server docs (resend.com/docs/mcp-server)
  3. Postmark MCP (ActiveCampaign) (github.com/ActiveCampaign/postmark-mcp)
  4. MailerSend MCP (developers.mailersend.com/mcp)
  5. AgentMail MCP (docs.agentmail.to/integrations/mcp)
  6. Twilio MCP server (www.twilio.com/docs/ai/mcp)
  7. Resend MCP (resend.com/mcp)

Facts are taken from each vendor's own pricing pages, documentation or terms as of the updated date above; prices are in US dollars per month unless stated. Cost comparisons apply each vendor's published rates to the same volume. MarkRelay is not affiliated with the providers covered here;MarkRelay sells a self-hosted email app, which this page mentions where it fits. Spotted something outdated? Tell us.

Revisions

  • : First published; every fact checked on the vendor's own pages.

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