Blueyonder are complete F***wits.
They have introduced some antivirus thing or another, which is clearly configured incorrectly, because so far all it has bounced has been innocent mail from a MODERATED mailing list that I have been on for about six years and that doesn't allow attachments. Now forgive me if I am being naive, but how in the name of the devil's illegitimate spawn can plain text email with no attachments be spreading viruses? More importantly when will the "tech" support tards at Bluechunder get a clue as to how to run a system? FFS