I noticed that when I have a mail, signed from a trusted person (means I have his PGP-key imported and signed), the signature is *never* shown as valid. An encrypted mail can be decrypted, so this works. My assumption is that the spam-filtering via Bogofilter breaks the signature - possible? Reproducible: Always Steps to Reproduce: 1. receive signed message 2. signature not shown as valid, although key is signed 3.
Easy x-check: If the mail is first read on my Android phone (while KDE is not active), the spam-filter does not grab it and attach the 'bogofilter score' later on when KDE is started. In this case the signature is displayed as 'green' ( valid). ...and I noticed that due to spam filtering the mail is always fetched twice from the mail server (the android always rings twice....)
I can confirm that this is still happening with KMail 4.11.1. The signature is actually broken by any mail filter with Pipe Trough action. E.g. mail filtering replaces Content-Type: text/plain; charset=iso-8859-1 with Content-Type: text/plain; charset="iso-8859-1" thus breaking the GPG signature.
*** This bug has been marked as a duplicate of bug 331991 ***