In the category of annoying defaults, I discovered that Postfix queries the DNS server directly and ignores /etc/hosts. Add the following to your /etc/postfix/main.cf to use the system defined look up order (in /etc/nsswitch.conf on Linux).
smtp_host_lookup = native
I discovered this because my new mail server has to relay mail to my old mail server while I migrate to the new one. The old server is on a private IP but is receiving mail for a sub-domain which resolves to a public IP. The simplest solution was to add the domain to /etc/hosts temporarily. Well, it would have been the simplest, if it had worked immediately. ^_^