As I understand it, it would also mean that the recipients PC would send out a 'read' receipt, which would trigger a notification of outgoing mail from some AV software. I can only speak of Norton, as I've not used anything else, but that sort of thing can be slightly worrying, as you appear to be sending mail out unexpectedly, which could be the symptom of a virus. I suppose the MSGTAG footer would explain that, but, to me, it would still be irritating.