It has been reported to me that messages signed with claws-mail are reported to have bad signatures by other clients (such as Mutt or Thunderbird). En example of a message produced by claws-mail can be found here: http://richard-falken.com/files/374 I have been told by many people this source is not valid. Encrypted messages are also built in a not valid manner.
As far as we know, we strictly adhere to GPG and MIME protocols. It would be nice that the many people could point out what it wrong in the pointed email in technical terms.
(Please reopen with technical information)