Question : How Can I Trace Email Routing Failure?

I am a webhosting reseller, and also network admin for small business.  My business email is handled through Microsoft Exchange Server 2007 on in-house hardware.  Spam and Anti-virus is handled by Google's Postini service.  My webhosting server recently changed their IP address, but I'm not sure if that has caused the problem I am currently having:

I can receive emails to my business email account from most senders (as far as I know)  However, when I send a message from a test account for one of my hosted domains, I do not receive it in my business email.  I have a Gmail account, and when I test messages from my webhosted domains, they are delivered to Gmail with no problem, so I know the outbound mail server is working OK, and so my webhosting server provider is through assisting me with this issue.

I have conducted all of the SMTP flow tests at the Postini System Manager for my business account.  All test emails from there are received in my business Inbox.  My Exchange Server is sitting behind a firewall that blocks pings, so I can't really test anything that way, but since I am receiving most emails I don't think it has anything to do with my firewall.

I did notice one flaw in the DNS Zone file for my business domain - there is one duplicate line in my MX record, which I will have my provider change, but it seems unlikely that is my problem.

Can anyone help me figure out why my business email account is not receiving email messages from my webhosting mail server?

Thanks,
Rob

Answer : How Can I Trace Email Routing Failure?

inbound mail flow is controlled by the MX record for your domain
if there is an invalid value, a remote system could be attempting to deliver mail using that record
verify and remove any invalid MX records for your domain's DNS zone