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Appendix

Reference ID​

The reference_id is an identifier that you can use to track the email. It is a 24-character hexadecimal string that uniquely identifies the email message. If you do not provide a reference_id, one will be automatically generated for you.

If provided, it must be a valid 24-character hexadecimal string.

Refer to the Reference ID documentation for more details on how to generate and use this ID.

Tags and Headers​

The tags and headers fields are optional. The maximum key length is 128 characters, and the maximum value length is 768 characters. Tags can be used to categorize the email, while headers can be used to add custom metadata to the email.

Tags are particularly useful for tracking and analytics, and work well with our webhooks API.

Template Data​

When sending templated emails, you can include dynamic variables, objects, and arrays in the template_data object. These can be used to personalize the email content based on user-specific data and work seamlessly with the templating engine.

Scheduled Delivery​

If you include the scheduled_at field, the email will be queued for delivery at the specified time. This can be provided in RFC 3339 format (e.g., 2024-08-05T11:52:01.858Z) or in natural language (e.g., tomorrow at 2pm, in 2 hours).

If omitted, the email is sent immediately. Scheduled emails count toward your sending limits only at the time of delivery. You cannot schedule emails more than 21 days in advance, and no more than 25% of your monthly limit can be pending as scheduled at any time.

Warning: Scheduled delivery is not supported for bulk emails endpoint. This feature is only available for basic & templated emails.

Inline Images​

You can include inline images in your email by specifying the inline field in the attachments array. When set to true, the image will be embedded in the email body and can be referenced using a cid (Content-ID) in the HTML content as shown below:

<img src="cid:my-image.png">

Limitations​

  • The maximum size of the HTTP request body is 34,865,152 bytes (approximately 34.9 MB).
  • The total size of all attachments in an email cannot exceed 26,214,400 bytes (approximately 26.2 MB).
  • The maximum number of attachments per email is 32.
  • The maximum number of recipients (to, cc, bcc) combined is 500.
  • For bulk emails, the maximum number of recipients per request is 500. This refers to the messages array of the bulk endpoint, not the individual to, cc, or bcc fields.