Exchange Sever 2010 Stops Working After Few Days.

Hi,

We have an Exchange 2010 server running on Windows 2008 server R2 (DC). We had another server running Windows 2003 server SBS and Exchange 2003 (DC, master of all FSMO roles). The old server (2003) died so we decided to seize all the roles to the current 2008 server. Now every few days the Exchange server stops sending/receiving emails from/to outside. Internal emails are OK. In the "queue viewer" it shows all the sent emails are waiting to be delivered with the "Retry" message. When I restart the server everything comes back and the server sends all the messages it had on its queue but after few days, it starts all over again and I have to restart it again which is frustrating.

When the Exchange is down (before restart) people who sent emails get a reply email from the server after few hours as below:

Reporting-MTA: dns; DC1.contoso.local

Final-recipient: RFC822; david@yahoo.com
Action: failed
Status: 4.3.2
X-Supplementary-Info: < #4.3.2 smtp;550 4.3.2 QUEUE.Admin; message deleted
 by administrative action>

Could you please help me. It cause problem to our whole network.

Note: Server is DC, GC, has sized all the FSMO roles, DNS, DHCP, Backup Server, AV server (Trend Micro), Exchange 2010 server, File Server, Web Server.

Its utilization is low as we have only 5 employees.