Exchange 2003 — Slow Sending/Receiving

We have Windows 2003 Server that is our DNS, DC, and Exchange Server(I know this is not recommended but you use what you have). We have been experiencing signifcant slow downs in email delivery internally. It only happens once maybe twice a day. Also, our “Outlook 2003 users recieve an error that says Outlook is trying to contact EXchangeServer.domain.local.”

Any ideas on where I can start troubleshooting and what I could look at first would be greatly appreciated.
Thanks