Outbound Messages Awaiting Delivery - Host Unreachable

[qman]

I currently have ...
Exchange Server 5.5 SP4
ISDN Dial-up to ISP with "Forward all messages to Host" ticked incl. IP address

In the last week, most messages (especially with attachments, but not always) just sit in the Outbound Messages Awaiting Delivery Queue. The recipient has [Host Unreachable] after the address. I can ping and perform nslookups for the required outbound addresses, and occasionally mail gets through. All mail is getting in fine. It is a very simple setup and has me baffled.
Any help would be appreciated!

 

[Xybertron]

Why forward? Just so I know.

Have you tried putting [x.x.x.x] rather than just the ip of the place you want to forward to? This causes no lookups and should help.

Dan
Microsoft Exchange Support @ Microsoft