PGP signatures as email attachments

I run Eudora 5.1 on a Windows 2000 machine. When someone attempts to verify their identity by including their PGP signature with an email message, it comes through as an attachment. Maybe I just use old email clients. Given that more and more organizations are banning attachments, or at the very least urging folks not to open them, I'm not sure how the sender's use of a PGP signature helps authenticate himself. This is an issue I'll be digging into next week at Inbox.