now is the time to forget the whimpering child, become the warrior
10.12. Enigmail sees some emails as broken
This problem occurs often when using an IMAP mailserver and is due to Thunderbird / SeaMonkey not downloading the message as a whole. To fix this problem, go to OpenPGP → Preferences → Advanced and disable the option Only download attachments when opened (IMAP only).
See also the explanation of this option in Section 9.1.4.