The masquerade domain replaces any local domain name used in any
Mail From lines in the protocol. The replacement occurs on
the first hop only.
The Mail From line refers to the SMTP message heading
information. It is the address NDRs will be sent
to, and is not the From line displayed by the receiving
client. The From line displayed by e-mail clients is in the
message body, which a masquerade domain name does not affect.