Email problems

Ok, I've been having problems with client email on Mac OS 10.3 and 10.4. Let me explain further... We have a Windows 2003 Domain Controller running DNS and DHCP. Entourage would give an error message saying that it could not send mail out, but it could receive... I did some troubleshooting, resetted the PRAM, check the settings (and they were correct), deleted the entourage preferences, created different profile and same problem... It all solved when instead of havinf DHCP under the network settings I assigned an IP manually and it worked... Now I had the same problem with the mail client biult-in in Mac OS 10.4. I could receive mail but not send out... I did the manual IP configuration and it worked again... Can somebody explain what is going on and how can I fix this so it would work with DHCP.... Thanks