[RobertGraham]

Rules are always viewed from the point of connection initiation. That is,

A -> B XLATE C -> B

means that whenever A opens a connection to B the addr will be xlated. The packet on the return will also be xlated since the technology is stateful. This is what you are seeing.

However, if B wants to initiate a connection to C and have the packet end up at A, the rule above isn't sufficient. As long as the mail server doesn't need to initiate a connection to this other network - you're fine. In the case of an IMAP server that doesn't need Internet access and isn't the SMTP server is a perfect candidate for this one way static NAT situation.

Does that clear up the confusion?