This is some intermittent error that seems to occur. It would help if you could send us one of those messages. In Mail, select that message and do a “Save As…”, then select “Raw Source”. You can send it to support@devon-technologies.com.
This would help me to find out what’s so special about that message that it triggers this problem.
Update The “-[NSCFString count]: selector not recognized” problem has been reproduced and will be fixed in the next release (kudos to Scott Morrison of MailTag fame!).