I have a machine with mutliple aliases on it. We'll say ext_if="fxp0" (192.168.1.1) and alias_if="192.168.1.2". net_gw="192.168.1.254" is the gateway.
There is an smtpd on $alias_if that accepts mail. However, when mail is sent from the host, the source address of any connections is $ext_if (as to be expected).
Recommend that you fix this at the daemon level. I have the very same setup you describe. By tweaking the MTA, the outbound connections now get sourced from the alias IP.
.joel
