Changes to Email API validation: Reminder -2nd Blackout period planned for November 15th, 5pm - 7pm EST

Wednesday, 15 November 2 hours

The maintenance is now complete. Thanks for your patience.


The scheduled maintenance is now underway. We'll keep you updated on our progress.


Changes to Email API validation – Action may be required

We're making some improvements to our API infrastructure and as a result, we're changing how we handle the validation of your API requests.

In the past, Postmark's email API has been pretty forgiving to requests that are passed incorrectly. In some cases, we've accepted invalid request headers, malformed JSON and mismatched field types — even if they didn't match our API documentation. Going forward, we'll start to reject invalid requests and return an error message instead.

If you see Postmark return error messages to API requests that were previously accepted without issues, please double-check your request JSON and ensure it is actually valid. The possible failures are:

  • HTTP error 415: Unsupported Media Type. Solution: Check that the documented request headers are being passed (Content-Type and Accept)
  • HTTP error 422. API error code: 403 — Invalid request fields(s) - {FIELD_NAME}. The {FIELD_NAME} will indicate where the invalid JSON is.
  • HTTP error 422. API error code: 402 — Invalid JSON. The request payload is not valid JSON

We recommend using a tool such as JSONLint to check your request payloads for any obvious validation errors, if that shows no issues then refer to our API documentation pages and make sure your request matches the schema(s) we are expecting.

API request validation will be fully enforced starting on Tuesday, November 28th but we’ll be running a set of blackout periods before that to help you catch any issues. Here’s what to expect:

  • November 15th: Blackout period From 10:00am -12:00pm EST (2 hours)
  • November 15th Blackout period from 5:00pm - 7:00pm EST (2 hours)

See our full announcement here for more details and some common reasons why your request might be invalid.

If you have any questions or need any help let us know.

Began at:

Affected components
  • API