Lettr for AI agents

Lettr is a transactional and marketing email platform with templates, audiences, campaigns, and delivery tools for sending at scale from one dashboard. Connect it once through Arc0, and your agent, or Claude, ChatGPT and Cursor, can use it through one MCP endpoint, limited to what each user approved.

Connect Lettr See the 60 actions ↓

EmailAPI keyMCP + RESTlettr.io

AUDIT LOG · LETTRPOLICY: acme-support

09:41:07 · claude · u_8f2read

lettr.list_emails

List Sent Emails✓ allowed · 212ms

09:41:08 · claude · u_8f2read

lettr.get_email

Get Email✓ allowed · 164ms

09:41:09 · claude · u_8f2write

lettr.create_domain

Create Domain✓ approved · approved by user

09:41:12 · claude · u_8f2destructive

lettr.delete_domain

Delete Domain✕ blocked · policy: deny

EVERY LETTR CALL, ON THE RECORD

01 · USE CASES

What agents do in Lettr.

Add a contact to an audience

Add a new contact to an audience list after they opt in on a signup form.

Check a scheduled email's status

Look up a scheduled email to confirm it will send at the right time.

Approve before deleting a domain

Require approval before deleting a sending domain, since it would break outgoing mail.

02 · ACTIONS

60 Lettr actions, graded by risk.

Every Lettr action is tagged read, write or destructive, so one policy covers the whole app and new actions inherit the right default.

read

Look things up. Allowed by default.

lettr.get_email
Get Email
lettr.get_domain
Get Domain
lettr.get_webhook
Get Webhook
lettr.list_emails
List Sent Emails
lettr.get_campaign
Get Campaign
lettr.get_template
Get Template
lettr.list_domains
List Domains
lettr.list_projects
List Projects
lettr.list_webhooks
List Webhooks
lettr.list_campaigns
List Campaigns
lettr.list_templates
List Templates
lettr.get_audience_list
Get Audience List
lettr.list_email_events
List Email Events
lettr.get_audience_topic
Get Audience Topic
lettr.get_scheduled_email
Get Scheduled Email
lettr.list_audience_lists
List Audience Lists

+ 9 MORE

write

Create and change things. Allow, or ask the user first.

lettr.send_email
Send Email
lettr.create_domain
Create Domain
lettr.send_campaign
Send Campaign
lettr.create_webhook
Create Webhook
lettr.update_webhook
Update Webhook
lettr.create_template
Create Template
lettr.update_template
Update Template
lettr.create_audience_list
Create Audience List
lettr.update_audience_list
Update Audience List
lettr.create_audience_topic
Create Audience Topic
lettr.update_audience_topic
Update Audience Topic
lettr.create_audience_contact
Create Audience Contact
lettr.create_audience_segment
Create Audience Segment
lettr.update_audience_contact
Update Audience Contact
lettr.update_audience_segment
Update Audience Segment
lettr.create_audience_property
Create Audience Property

+ 8 MORE

destructive

Delete, cancel or archive. Ask first, or deny outright.

lettr.delete_domain
Delete Domain
lettr.delete_webhook
Delete Webhook
lettr.delete_template
Delete Template
lettr.delete_audience_lists
Delete Audience Lists
lettr.delete_audience_topic
Delete Audience Topic
lettr.cancel_scheduled_email
Cancel Scheduled Email
lettr.delete_audience_contact
Delete Audience Contact
lettr.delete_audience_segment
Delete Audience Segment
lettr.delete_audience_property
Delete Audience Property
lettr.get_template_merge_tags
Get Template Merge Tags
lettr.detach_contacts_from_lists
Detach Contacts from Lists

03 · HOW IT WORKS

Lettr in three steps.

01Your users connect LettrThey add their Lettr api key on Arc0 Connect, under your brand. It goes straight into the vault.
02You set the rulesReads run, writes like “create Domain” can wait for the user, and “delete Domain” can be denied outright.
03Any agent can actYour agent calls Lettr through the Arc0 SDK or MCP, and so can Claude, ChatGPT and Cursor. Every call lands on the audit log.

POLICY.TS

await arc0.policies.set('lettr', {
  read: 'allow',
  write: 'ask',        // create_domain
  destructive: 'deny',  // delete_domain
})

# Claude Code: the same connection, one URL
$ claude mcp add --transport http arc0 \
    https://mcp.arc0.ai/u/u_8f2

04 · AUTH AND DATA

How Lettr connects.

Users add their Lettr api key on Arc0 Connect. It is encrypted in the vault, never shown to the model, and each user can rotate or revoke it at any time.

The same Lettr connection serves your agent over MCP and your own backend over REST and the proxy, so a user connects once. How Arc0 handles credentials →

AUTH: API key
CREDENTIALS: Per-tenant encrypted vault
MODEL SEES: Results only, never credentials
AUDIT LOG: Every call, on every plan

05 · WORKS WITH

Use Lettr from any agent.

Claude

ChatGPT

Cursor

Codex

VS Code

OpenAI Agents SDK

Claude Agent SDK

Vercel AI SDK

Mastra

LangGraph

06 · RELATED

More email apps.

Elastic Email91 actions

07 · FAQ

Lettr and Arc0, answered.

Q01

Can I use Lettr with Claude, ChatGPT or Cursor?

Yes. Connect Lettr to Arc0 once, then add your Arc0 MCP URL to Claude, ChatGPT, Cursor, Claude Code or any other remote-MCP client. Each assistant only gets the Lettr actions you allow.

Q02

How do users connect Lettr?

Users add their Lettr api key on Arc0 Connect. It is encrypted in the vault, never shown to the model, and each user can rotate or revoke it at any time.

Q03

Which Lettr actions can my agent take?

60 in total: 25 read, 24 write and 11 destructive, such as “create Domain”. Your policies decide which of them each agent may call.

Q04

Can I stop my agent from deleting things in Lettr?

Yes. Actions like “delete Domain” are graded destructive. Set destructive actions to deny, or to ask so the user approves each one, and blocked calls still show up on the audit log.

Q05

Can my own backend call Lettr too?

Yes. The same Lettr connection is available over REST and through the Arc0 proxy, so your product and your agent share one connection per user.

Get started

Plug Lettr into your agent.

Your users connect Lettr once, under your brand. Your agent gets 60 actions behind your policies, with every call on the record.

Connect Lettr Browse all apps →

Free to build · MCP + REST · Audit log on every plan