I'm having trouble reading signed emails (in the mail app, not gmail).
The email client only shows: "mail is still on server" message, not the email content.
As far as I know, this is simply a bug in the HTC email application. You can try using a different client (K9 supports signed messages, I believe), but beyond that you'll have to take it up with HTC and hope that they decide to fix it at some point.
Here's a related XDA thread where someone includes what he says is a response from HTC regarding the issue. There's also a thread on the Android Central forums where someone reports the same bug (and no solution). Those threads are a little on the old side, but I don't believe HTC has fixed this yet.