When you send a MIME attachment to a POP3 recipient and you have this recipient configured to Always send to this recipient as RTF, the attachment is not received.

Most POP3 clients do not understand RTF (Rich Text Format) in Microsoft TNEF (Transport Neutral Encapsulation Format).

Either disable the Always send to this recipient as RTF option for POP3 clients or don't send an attachment in a RTF message.

NOTE: I believe COMPUSERVE recipients receive these attachment, but can't decode them (so don't send one to me).