Corrupted Attachments or Incomplete Folder Downloads
Some IMAP servers don't work well with a particular hidden Postbox setting. If you are receiving attachments that appear to be corrupted:
1) Go to Preferences / Advanced / General / Config Editor on the Mac or Tools / Options / Advanced / General / Config Editor (on Windows), then locate the search bar at the top of the window.
2) paste in mail.server.default.fetch_by_chunks
3) double-click to change the value to false
4) paste in mail.server.default.mime_parts_on_demand
5) double-click to change the value to false
6) paste in mail.imap.fetch_by_chunks
7) double-click to change the value to false
8) Restart Postbox.
After restarting, you will want to rebuild the index for IMAP folders containing the message with the corrupt attachment.
1) Right-click the folder and then select Properties...
2) Click the Rebuild Index button.