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Having problems with DNS and kicked back messages

We have a bit of a strange setup and I need some help/advice!

Our Exchange is part of SBS 2003. It's the only server for this company...

We're using the built-in POP Connector to pull down email from their host ( because we didn't want to purchase a bunch of spam protection and a backup mx subscription. Their mail host is providing email for free and do all the spam/virus scanning before it hits the server.

However, we can't use SMTP forwarding because our provider requires SMTP authentication.

So... I setup and pointed it to our static IP. The MX record points to

Right now, I have the SMTP virtual server setup like this:

Masquerade domain:
Fully qualified domain name:

I have the external DNS addresses set to the ones our ISP provided.

At first we were getting kickbacks because I hadn't setup yet and was trying to totally masquerade as (including putting that into the FQDN field), but servers that did a reverse DNS lookup wasn't liking that the IP of the sender wasn't actually the IP address of Lesson learned. That is when I added the and had the ISP make the lookup correct. I've verified that it works at a reverse lookup now gives our static IP.

We still seem to get kickbacks from a few certain domains. Is it bad to have a message coming from when the MX record points to I didn't think it would matter, but I need the advice of an Expert.

If you need any other information, don't hesitate to ask... I will provide any details you may need!

Thank you very much in advance, Experts!
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I think for now I'll try to get the SMTP connector working with authentication.

I understand that in a perfect world, a "vanilla" Exchange configuration would probably be best, but the company is pretty much dead set against opening up the server to accept email directly. They feel safer behind the POP Connector, so while my thoughts about it may have been flawed, for now it's in their interest to keep email coming as is.

I removed the incorrect Masquerading information that you pointed out, and using SMTP forwarding should put any email delivery problems in the hands of the host... providing that the connector is working, of course!

Thanks for your links and information!