[Serusers] SIP_DOMAIN and bootup question

I've been looking through the admin manual and haven't found what I'm looking for yet. I point my SIP client to, say, domainname.com, and on my DNS server, I point this to my SER system's IP address, say, 10.10.10.10. When I had SIP_DOMAIN=10.10.10.10, I experienced long delays between pressing hangup and the call actually being hung up. When I changed this to SIP_DOMAIN=domainname.com and ran /etc/init.d/ser restart, the behaviour is as expected. I put export SIP_DOMAIN=domainname.com into /etc/profile and reboot the system, I try dialing and then hanging up again, and it's back to the same behaviour I mentioned above. If I run /etc/init.d/ser restart, then it is fixed again. Am I doing something wrong here? I don't want to have to run a manual restart after every system boot. Thanks in advance.