SpamSieve mangels messages

Since I have installed spamsieve several mails have arrived mangeled. Today I even received an e-mail with the sender missing. Well, to be honest I’m not sure whether it’s really spamsieve that mangels the mails, but it didn’t happen before I installed it. And:
This only happens on the computer where spamsieve is installed, another computer accessing the same imap account shows correct mails.

The way SpamSieve integrates with the mail program, it’s not even possible for it to modify the messages. So there must be some other cause. It sounds like maybe your mail program’s database is damaged and needs to be rebuilt.