Symantic AV spits that out. Unfortunately Symantic will not say what exactly constitutes a "Malformed Container". Try zipping the attachment and also make sure that the email text does not have any lines starting with a double hyphen "--", that has been known to trigger the error.
It has nothing to do with PZ, its all Symantic.
edit:
use this link. It may give you a bit of info on what to change in your emails
http://www.upenn.edu/computing/virus...mtp_3.1_ig.pdf
of interest is #4 setting your blocking policy.