Area:
Exchange 2010, IMAP, Thunderbird

Synopsis:

In Thunderbird, there is a problem where attachments on received e-mail messages are corrupted, and images in e-mail messages only partially load. This is caused by Exchange reporting attachment sizes incorrectly, so when Thunderbird breaks the message into chunks it doesn’t download all of them.

Solution:
Exchange has a bug in the IMAP server implementation that reports the wrong size for messages and attachments. There is a workaround that causes Thunderbird to ignore the erroneous information.

Truncated Attachment Problem
Warning: This may result in any tags you have set disappearing. If you use Thunderbird’s tagging you should back up your profile before doing the following (your profile is in the ~/.mozilla-thunderbird directory by default).

  • From the Edit menu, select Preferences
  • Select the Advanced tab
  • Click the Config Editor button
  • Set the filter to mail.server.default.fetch_by_chunks. The default value should be true
  • Double-click the mail.server.default.fetch_by_chunks preference. It should turn bold, and the value should now read false

Thunderbird will now download attachments as one block, avoiding the bad attachment size problem.