Default Mail does not load GPG-signed messages

Some messages are not loaded from the server (IMAP). The message window says:
[This mail is still on the server]
No error message or any indication of what goes on. The "unreadable" messages can be read with other clients. I have examined the raw message content with headers, and the only thing these messages seem to have in common are that they contain a GPG signature. Does HTCs mail-application reject signed messages? Shouldn't it at least display the unencrypted message body?