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Why doesn’t the attachment content of my inbound message show up in Activity?

If there was an attachment included in an inbound message, there will also be a Content field that includes the attachment’s base64 encoded content when we post the inbound message to your URL. We send this in the JSON we post to your webhook URL but it will not show in the JSON tab in your Activity page for the inbound message.

You can see the base64 we sent in the Raw Source tab, however. Depending on the size of the attachment, some of the base64 might be omitted in the Raw Source tab but the complete attachment base64 bytes are sent in the JSON we post to your webhook URL.

As an example, the Attachments field will have this form when we POST to your inbound webhook URL:

    "Content":"[BASE64-ENCODED CONTENT]",
    "ContentLength": 4096,
    "Content":"[BASE64-ENCODED CONTENT]",
    "ContentLength": 16384,

If you want to confirm we are sending the attachment bytes in the Content field, you can quickly set up a Request Catcher and use the URL it generates for you to inspect a POST from us.

  1. Create a new URL
  2. Set the URL it generates for you as your inbound webhook URL in your Inbound Stream settings in Postmark (Inbound Stream > Settings)
  3. Send an email with an attachment to your inbound email address
  4. Inspect the POST you receive at your Request Catcher URL to see the base64 bytes in the Attachments array’s “Content” field.

Keep in mind that Postmark’s inbound message size limit is 35 MB.

Last updated June 11th, 2020

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