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Q: Why is Exchange creating ATT00001 attachments?

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The problem:

  • When I format a message with an in-line attachment, such as a screen shot, followed by more text, everything after the first attachment gets displayed as another attachment.
  • The generated attachment file have names similar to 'ATT00001'

What causes it:

  • This happens if you tried to send a message that intermingles text sections and attachment sections.
  • This can happen to any messages that pass through MIT's Exchange server
    • MIT currently has Microsoft Exchange server version 2013
  • This is not dependent on the mail program, it can happen with any mail program
    • It is often seen with Apple Mail because of the flexibility Mail gives you in message format


This happens because Microsoft Exchange Server is reformatting messages sent through it.

The Exchange server insists that message text must always be first and attachments must always be last. As soon as the Exchange software sees one attachment in a message, it stops looking for text, and treats anything else in that message as an attachment. Any remaining text sections are converted into attachment sections, and given fake file names (like "ATT00001.htm").

At this time, there is no way to make the Exchange server stop turning text sections into attachment sections. The only workaround is to include attachments at the end of a message, after text and after any signatures.


Apple Mail

  • In Apple Mail, go to the menu Edit > Attachments
    • Make sure the following settings are checked:
      Always Send Windows-Friendly Attachments
      Always Insert Attachments at End of Message
  • Use the menu Mail > Preferences and go to the Composing section
    • Make sure that Message Format is set to "Plain Text"

With these settings, any attachments added with the "Attach" paperclip button will be put at the end of your message, after all message text.

Apple Mail still allows you do drag and drop attachments - if doing this, be sure to drop them at the end of the message. Attachments dropped in the middle of the message will cause any text that follows the attachment to be converted into another attachment.

Other Programs

  • To keep Exchange server happy, make sure that text sections occur before other attachment sections.

For more information

Microsoft has a KB article confirming that this is a problem with Exchange Server 2007 and 2010:


Documentation and information provided by the MIT Community

Last Modified:

April 23, 2020

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