SBS 2003 Exchange email relay problem over VPN

One of my users connects to a SBS 2003 box over VPN (SBS Box runs the VPN server and they obtain local IP on LAN subnet) and uses Exchange through Outlook.

Recently, to some. but not all domains they have been getting error code 5.7.1 (unable to relay). In the relaying restrictions in Exchange i have the whole local LAN subnet as well as our WAN Subnet. I have also checked the DNS servers where we use ours and the ISP's.

The user in question has also been able to regularly email this address in the past with no problems.

Can anyone point me in the right direction on how to resolve this?!

Thank you.
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cam-eAuthor Commented:
It ended up been that our servers were listed on black lists and have since been removed. thanks!
do a test with telnet if the users faces the same issue
Is this issue sphoradic
cam-eAuthor Commented:
No, same few email address and the user has tried each day for the last few days.

Yes, can telnet to the mail server from our mail server no problems.
cam-eAuthor Commented:
The ip address of our email server was listed on various spam filters.
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