Lettermint
Lettermint Sending API
Lettermint Sending API

Schemas

Download schema

ApiError

Structured API error.
​object · required

ErrorMessage

Error with only a message.
message
​string · required

Error overview.

MessageAttachmentInput

A file attached to the message.
filename
​string · required
content
​string · required
content_type
​string | null · maxLength: 255

The MIME type of the attachment. Supports parameters (e.g. text/calendar; method=REQUEST). If omitted, detected automatically from content or filename.

Example: application/pdf
content_id
​string | null · maxLength: 255 · pattern: ^[a-zA-Z0-9._@-]+$

Content ID for inline attachments, referenced via cid: in HTML body. If no @ is present, @lm is appended automatically.

Example: logo-image

MessageTagInput

A reusable exact-match tag with a name and a value.
name
​string · minLength: 1 · maxLength: 32 · pattern: ^[A-Za-z0-9_-]{1,32}… · required
value
​string · minLength: 1 · maxLength: 64 · pattern: ^[A-Za-z0-9_-]{1,64}… · required

PendingSendMailResponse

An accepted message that is queued for delivery.
message_id
​string · required
status
​string · const · required
Const value: pending
sandbox
​boolean · const

Present when the project uses Sandbox delivery

Const value:
sandbox_result
​SandboxResult · enum

The simulated delivery result. Present when the project uses Sandbox delivery

Enum values:
delivered
hard_bounced
soft_bounced
deferred
failed
suppressed
spam_complaint
auto_replied

SandboxResult

string · enum
Enum values:
delivered
hard_bounced
soft_bounced
deferred
failed
suppressed
spam_complaint
auto_replied

ScheduledSendMailResponse

An accepted message that is scheduled for later delivery.
message_id
​string · required
status
​string · const · required
Const value: scheduled
scheduled_at
​string · required

The scheduled delivery time in ISO 8601 format

sandbox
​boolean · const

Present when the project uses Sandbox delivery

Const value:
sandbox_result
​SandboxResult · enum

The simulated delivery result. Present when the project uses Sandbox delivery

Enum values:
delivered
hard_bounced
soft_bounced
deferred
failed
suppressed
spam_complaint
auto_replied

SendBatchMailRequest

​SendMailRequest[] · minItems: 1
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
sandbox_result
​SandboxResult · enum

The result that a Sandbox project simulates for every recipient.

Enum values:
delivered
hard_bounced
soft_bounced
deferred
failed
suppressed
spam_complaint
auto_replied
Example: hard_bounced
​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. Send values as plain text: Lettermint encodes non-ASCII characters and repairs small formatting errors, such as line breaks, control characters and missing angle brackets. Header names must be printable ASCII without a colon.

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 · maxLength: 255 · pattern: ^[a-zA-Z0-9_-]+(?:\s…

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

Example: welcome-email
​MessageTagInput[] · maxItems: 20

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

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

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

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
sandbox_result
​SandboxResult · enum

The result that a Sandbox project simulates for every recipient.

Enum values:
delivered
hard_bounced
soft_bounced
deferred
failed
suppressed
spam_complaint
auto_replied
Example: hard_bounced
​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. Send values as plain text: Lettermint encodes non-ASCII characters and repairs small formatting errors, such as line breaks, control characters and missing angle brackets. Header names must be printable ASCII without a colon.

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 · maxLength: 255 · pattern: ^[a-zA-Z0-9_-]+(?:\s…

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

Example: welcome-email
​MessageTagInput[] · maxItems: 20

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

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

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

TlsPolicy

string · enum
Enum values:
opportunistic
enforced

ValidationError

message
​string · required

Errors overview.

​object · required

A detailed description of each field that failed validation.