Just because a mail has a signature in it about a (possible) av was used to scan it, doesn't mean it was scanned by that (or for that matter any other av).
If your friends can't accept certain mails, they should go talk to the admin/IT department because as it seems, they don't have things setup correctly.
I can easily create a custom signature where is says "This mail was scanned with Kaspersky, avast, Eset" and dozens of other scanners, but that doesn't mean the mail was scanned by any of them.