We have a customer that has a BM server with 2 NIC's...public address
(x.x.x.253) and a private address (x.x.x.253). All outgoing email
(through BM) and web traffic shows the source as the public .253
address. Their MX record is configured as a secondary public address on
the BM server (.242 public). How can I set it up so that outgoing email
and web traffic shows source as .242 (same as MX record)?

They already had a static NAT entry translating .242 (public) to .253
(private) on the BM server.