We're migrating our mail servers to new IPs on a different subnet (we're not happy about it, but our ISP gave us little choice). so we activated the second NICs in our mail servers and connected them to the separate subnet, with the plan to deac the NICs on the old subnet after a week or so. However, while the DNS was propagating, we were getting rejected mail from some remote servers that complained that our forward and reverse lookups didn't match because our mail was still going out the old IP.
http://www.postfix.org/postconf.5.html#smtp_bind_address saved our bacon. We told Postfix to use just the new IP, and the rejections stopped. So if anyone else has multiple IPs on the same server, but you want to force your mail out on a specific IP, smtp_bind_address is for you. :) Thanks, SteveJ
