Starting today, it seems like ClamAV blocks lots of mail refering to a "
MBL_144360.UNOFFICIAL" which is most certainlly a FALSE POSITIVE (Several other threads on internet discussing this as well right now...)
It's submitted to bugtracker:
http://bugs.contribs.org/show_bug.cgi?id=6541Any solutions should be published here as well, I think, so that as many as possible can apply solution ASAP.
Edit: For full discussion and/or solution, please see bugtracker...