How do I send calendar invites with Postmark?

Sending calendar invites through Postmark is very similar to sending any other kind of attachment.

Here is some example JSON for sending an .ics file as an attachment using the Postmark API:

{
  "From": "sender@domain.com",
  "To": "attendee@domain.com",
  "Cc": null,
  "Bcc": null,
  "Subject": "Test Email with ICS Attachment",
  "Tag": null,
  "HtmlBody": "HTML Body here.",
  "TextBody": "Text body is here.",
  "ReplyTo": null,
  "Headers": null,
  "TrackOpens": true,
  "Attachments": [
    {
      "Name": "invite.ics",
      "Content": "[Your base64 encoded .ics content here]",
      "ContentType": "text/calendar; charset=utf-8; method=REQUEST",
      "Disposition": "inline"
    }
  ]
}

Note that the “Content” field will need to be replaced with your .ics file's base64.

The only real difference to keep in mind here when sending calendar invites as attachments, rather than sending an image or file, is to set the ContentType for the attachment to "text/calendar; charset=utf-8; method=REQUEST".

One best practice for sending calendar invites is to include the calendar attachment, both inline and as an attachment. For a wider range of compatibility with calendar servers and email clients, include your attachment inline with 7-bit encoding, and as an attached file with base64 encoding.

Additional Resources

Internet Calendaring and Scheduling Core Object Specification