Messages delivered to PMDF by VMS MAIL can come from the local system or can have originated on a remote system connected via either DECnet MAIL-11 or PSIMail. In the latter case, the message's return address will probably not conform to RFC 822 addressing conventions in which case PMDF must translate the address to legal RFC 822 address. PMDF uses a mapping scheme to convert VMS MAIL addresses into acceptable RFC 822 addresses and vice versa.

This mapping scheme is necessarily rather complex. There are four individual conversions that must be performed. These are described in detail in the following subsections.