Send

Download schema

Send a single transactional email or a batch of them in one request, with attachments, custom headers and tags.

Send an email

POST

https://api.lettermint.co/v1

/send

Send an email › Headers

x-lettermint-token

string

Project API token for the project sending this email.

Idempotency-Key

string

Send an email › Request Body

SendMailRequest

from

string · required

to

string[] · minItems: 1 · required

subject

string · maxLength: 998 · required

route

string

cc

string[]

bcc

string[]

reply_to

string[]

scheduled_at

string · maxLength: 255

ISO 8601 or English delivery time. A time without a timezone uses UTC.

Example: tomorrow at 9am

object

Custom headers to include in the email. To preserve a submitted RFC Message-ID, include both Message-ID and X-LM-Preserve-Message-ID with a string value of true or 1. Header names and control values are case-insensitive; false or 0 retains replacement.

Example: {"Message-ID":"<ticket-123@example.com>","X-LM-Preserve-Message-ID":"true"}

object

Metadata to track with the email (not added as email headers).

Example: {"user_id":"123","campaign_id":"welcome-2025"}

tag

string | null

Tag to categorize and filter emails (alphanumeric, underscores, hyphens, spaces allowed).

Example: welcome-email

object[] · maxItems: 20

Reusable exact-match tags. Names must be unique and are case-sensitive.

Example: [{"name":"campaign","value":"welcome-v2"},{"name":"environment","value":"production"}]

settings

object | null

Per-email settings that override the selected route settings for this email only.

Example: {"track_opens":false,"track_clicks":true,"tls":"enforced"}

html

string | null · minLength: 3

text

string | null · minLength: 3

object[]

Send an email › Responses

anyOf

At least one variant must match. Multiple variants may match simultaneously.

Decision Table

Variant	Matching Criteria
	type = object · requires: message_id, status, scheduled_at
	type = object · requires: message_id, status

Properties for Variant 1:

message_id

string | null · required

status

MessageStatus · enum · required

Enum values:

scheduled

pending

queued

quarantined

suppressed

processed

delivered

opened

scheduled_at

string · required

POST/send

curl -X POST https://api.lettermint.co/v1/send \
  -H "x-lettermint-token: $LETTERMINT_PROJECT_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"from":"John Doe <john@example.com>","to":["recipient@example.com"],"subject":"Hello","text":"Sent with Lettermint."}'

Example Request Body

{
  "route": "route",
  "from": "from",
  "to": [
    "string"
  ],
  "cc": [
    "string"
  ],
  "bcc": [
    "string"
  ],
  "reply_to": [
    "string"
  ],
  "subject": "subject",
  "scheduled_at": "tomorrow at 9am",
  "headers": {
    "Message-ID": "<ticket-123@example.com>",
    "X-LM-Preserve-Message-ID": "true"
  },
  "metadata": {
    "user_id": "123",
    "campaign_id": "welcome-2025"
  },
  "tag": "welcome-email",
  "tags": [
    {
      "name": "campaign",
      "value": "welcome-v2"
    },
    {
      "name": "environment",
      "value": "production"
    }
  ],
  "settings": {
    "track_opens": false,
    "track_clicks": true,
    "tls": "enforced"
  },
  "html": "html",
  "text": "text",
  "attachments": [
    {
      "filename": "filename",
      "content": "content",
      "content_type": "application/pdf",
      "content_id": "logo-image"
    }
  ]
}

json

Example Responses

{
  "message_id": "message_id",
  "status": "scheduled",
  "scheduled_at": "scheduled_at"
}

json

application/json

Send multiple emails in a batch

POST

https://api.lettermint.co/v1

/send/batch

Send up to 500 emails in a single request. Each email in the batch supports the same parameters as the single send endpoint.

Send multiple emails in a batch › Headers

x-lettermint-token

string

Idempotency-Key

string

Send multiple emails in a batch › Request Body

object[] · minItems: 1

from

string · required

to

string[] · minItems: 1 · required

subject

string · maxLength: 998 · required

route

string

cc

string[]

bcc

string[]

reply_to

string[]

scheduled_at

string · maxLength: 255

object

Example: {"Message-ID":"<ticket-123@example.com>","X-LM-Preserve-Message-ID":"true"}

metadata

string[]

Metadata to track with the email (not added as email headers).

Example: {"user_id":"123","campaign_id":"welcome-2025"}

tag

string | null

Tag to categorize and filter emails (alphanumeric, underscores, hyphens, spaces allowed).

Example: welcome-email

object[] · maxItems: 20

Reusable exact-match tags. Names must be unique and are case-sensitive.

Example: [{"name":"campaign","value":"welcome-v2"},{"name":"environment","value":"production"}]

settings

object | null

Per-email settings that override the selected route settings for this email only.

Example: {"track_opens":false,"track_clicks":true,"tls":"enforced"}

html

string | null · minLength: 3

text

string | null · minLength: 3

object[]

Send multiple emails in a batch › Responses

Array of message IDs and their status

object[]

message_id

string · required

status

MessageStatus · enum · required

Enum values:

scheduled

pending

queued

quarantined

suppressed

processed

delivered

opened

scheduled_at

string

POST/send/batch

curl https://api.lettermint.co/v1/send/batch \
  --request POST \
  --header 'Content-Type: application/json' \
  --data '[
  {
    "route": "route",
    "from": "from",
    "to": [
      "string"
    ],
    "cc": [
      "string"
    ],
    "bcc": [
      "string"
    ],
    "reply_to": [
      "string"
    ],
    "subject": "subject",
    "scheduled_at": "scheduled_at",
    "headers": {
      "Message-ID": "<ticket-123@example.com>",
      "X-LM-Preserve-Message-ID": "true"
    },
    "metadata": {
      "user_id": "123",
      "campaign_id": "welcome-2025"
    },
    "tag": "welcome-email",
    "tags": [
      {
        "name": "campaign",
        "value": "welcome-v2"
      },
      {
        "name": "environment",
        "value": "production"
      }
    ],
    "settings": {
      "track_opens": false,
      "track_clicks": true,
      "tls": "enforced"
    },
    "html": "html",
    "text": "text",
    "attachments": [
      {
        "filename": "filename",
        "content": "content",
        "content_type": "application/pdf",
        "content_id": "logo-image"
      }
    ]
  }
]'

Example Request Body

[
  {
    "route": "route",
    "from": "from",
    "to": [
      "string"
    ],
    "cc": [
      "string"
    ],
    "bcc": [
      "string"
    ],
    "reply_to": [
      "string"
    ],
    "subject": "subject",
    "scheduled_at": "scheduled_at",
    "headers": {
      "Message-ID": "<ticket-123@example.com>",
      "X-LM-Preserve-Message-ID": "true"
    },
    "metadata": {
      "user_id": "123",
      "campaign_id": "welcome-2025"
    },
    "tag": "welcome-email",
    "tags": [
      {
        "name": "campaign",
        "value": "welcome-v2"
      },
      {
        "name": "environment",
        "value": "production"
      }
    ],
    "settings": {
      "track_opens": false,
      "track_clicks": true,
      "tls": "enforced"
    },
    "html": "html",
    "text": "text",
    "attachments": [
      {
        "filename": "filename",
        "content": "content",
        "content_type": "application/pdf",
        "content_id": "logo-image"
      }
    ]
  }
]

json

Example Responses

[
  {
    "message_id": "message_id",
    "status": "scheduled",
    "scheduled_at": "scheduled_at"
  }
]

json

application/json

Generic

Send an email

POST

https://api.lettermint.co/v1

/send

Send an email › Headers

x-lettermint-token

string

Project API token for the project sending this email.

Idempotency-Key

string

Send an email › Request Body

SendMailRequest

from

string · required

to

string[] · minItems: 1 · required

subject

string · maxLength: 998 · required

route

string

cc

string[]

bcc

string[]

reply_to

string[]

scheduled_at

string · maxLength: 255

ISO 8601 or English delivery time. A time without a timezone uses UTC.

Example: tomorrow at 9am

object

Example: {"Message-ID":"<ticket-123@example.com>","X-LM-Preserve-Message-ID":"true"}

object

Metadata to track with the email (not added as email headers).

Example: {"user_id":"123","campaign_id":"welcome-2025"}

tag

string | null

Tag to categorize and filter emails (alphanumeric, underscores, hyphens, spaces allowed).

Example: welcome-email

object[] · maxItems: 20

Reusable exact-match tags. Names must be unique and are case-sensitive.

Example: [{"name":"campaign","value":"welcome-v2"},{"name":"environment","value":"production"}]

settings

object | null

Per-email settings that override the selected route settings for this email only.

Example: {"track_opens":false,"track_clicks":true,"tls":"enforced"}

html

string | null · minLength: 3

text

string | null · minLength: 3

object[]

Send an email › Responses

anyOf

At least one variant must match. Multiple variants may match simultaneously.

Decision Table

Variant	Matching Criteria
	type = object · requires: message_id, status, scheduled_at
	type = object · requires: message_id, status

Properties for Variant 1:

message_id

string | null · required

status

MessageStatus · enum · required

Enum values:

scheduled

pending

queued

quarantined

suppressed

processed

delivered

opened

scheduled_at

string · required

Send multiple emails in a batch

POST

https://api.lettermint.co/v1

/send/batch

Send up to 500 emails in a single request. Each email in the batch supports the same parameters as the single send endpoint.

Send multiple emails in a batch › Headers

x-lettermint-token

string

Idempotency-Key

string

Send multiple emails in a batch › Request Body

object[] · minItems: 1

from

string · required

to

string[] · minItems: 1 · required

subject

string · maxLength: 998 · required

route

string

cc

string[]

bcc

string[]

reply_to

string[]

scheduled_at

string · maxLength: 255

object

Example: {"Message-ID":"<ticket-123@example.com>","X-LM-Preserve-Message-ID":"true"}

metadata

string[]

Metadata to track with the email (not added as email headers).

Example: {"user_id":"123","campaign_id":"welcome-2025"}

tag

string | null

Tag to categorize and filter emails (alphanumeric, underscores, hyphens, spaces allowed).

Example: welcome-email

object[] · maxItems: 20

Reusable exact-match tags. Names must be unique and are case-sensitive.

Example: [{"name":"campaign","value":"welcome-v2"},{"name":"environment","value":"production"}]

settings

object | null

Per-email settings that override the selected route settings for this email only.

Example: {"track_opens":false,"track_clicks":true,"tls":"enforced"}

html

string | null · minLength: 3

text

string | null · minLength: 3

object[]

Send multiple emails in a batch › Responses

Array of message IDs and their status

object[]

message_id

string · required

status

MessageStatus · enum · required

Enum values:

scheduled

pending

queued

quarantined

suppressed

processed

delivered

opened

scheduled_at

string