Message not being sent (unreachable destination)

[NJJ]

I have SBS 2000 with SP2 and Exchange SP2.
Using SMTP mail, any external mail being sent gets stuck in the 'messages with an unreachable destination' queue. Reapplying the internet connection wizard or rebooting clears the queue for a while but within hours it's stuck again. Incoming mail is fine.
Any ideas?

 

[Xybertron]

DNS point only to internal and forwarders to internet? Dan
Microsoft Exchange Support @ Microsoft

 

[NJJ]

The DNS is set up to internal and forwarders to internet. The SMTP mail is forwarded to a smart host. smtp.xxx.com
It sends ok for a while and then the messages just get stuck in the 'messages with an unreachable destination' queue. Starting and stopping the exchange service gets it going again.