strange responses from mails to the list

Hi! Now I get strange responses from some listmembers, because their mailfilters are setup too stron or there is again a nice kiddy threating this list, like: [bla bla] This is the mailserver xy from xy.net Your mail has been rejected of the following reason: ... [bla bla] Ther was no f-word or anything other bad on it to get such a reply (or something with their filters is wrong). Is there a possibility to change to pgp to avoid this and other spam? Philippe