I am not sure what changed in v10.5.3, but after updating, my PHP scripts would no longer send out mail, even though Apple mail was fine. Maybe a fluke, maybe note. But it looks like the domain names wouldn't resolve after the update. This fixed it:
Open up a terminal window
Type this: sudo vi /etc/resolv.conf
add a couple of name servers
Save and exit.
Now try mailing again. I hope this saves someone a couple hours of frustration.
[edited by: coopster at 5:46 pm (utc) on June 7, 2008] [edit reason] removed specific nameservers [/edit]