What are the Attachment and Email Size Limits?

Emails and attachments have size limits for both sending outbound messages through Postmark or receiving messages as JSON with inbound processing. This help article will make you aware of what size limits are in place for both outbound sending and inbound processing, which will be useful for troubleshooting sending or inbound processing errors and advising your application’s users of best practices.

Sending Size Limits

  • TextBody and HtmlBody can each be up to 5 MB each
  • Total message size, including attachments, can be 10 MB
If you attempt to send an email that exceeds any of these size requirements, the message will be rejected. We calculate the size of the email after it has been base64 encoded, so please keep in mind your final email size may be larger after being encoded.

Inbound Size Limits

  • Total cumulative size for all Inbound attachment files may not exceed 35 MB
To check the total size of an inbound webhook notification, you can use the Content-Length header of the POST request received. Now that you know the size limits in place, you can advise your users of best practices around sending and receiving emails with your application.
Last updated April 19th, 2019

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