Just surfboarding this topic, I have email correspondents whose spam filters are so neurotic/paranoid, that no attachments whatsoever are accepted. Never mind that the Word doc is from a squeaky-clean MacOS, or the .jpg pics processed via a Mac, no way will the a-v program allow them through. The workaround has been using .pdf.
Another thing I've found to cause incompatibility and triggering of spam filters on WinOS machines is the email application. I can't fathom why a plain text email from Eudora is classed as spam by OLExp, or as containing a 'virus' by AVG, for example.