QUESTION POSED ON: 12 September 2008
Our incoming and outgoing email goes through an offsite spam/virus/content filter. We are using network address translation (NAT) to map our internal network (Exchange 2000 Server) to an external public IP address. All SMTP traffic should flow between the external IP of the Exchange server and the offsite IP address. Our MX record points to the offsite address. The Sender Policy Framework (SPF) is configured with the offsite IP. Email traveling this route goes to all domains including those using SPF authentication.
Everything I've mentioned above is working fine. We are running a sniffer on our network gateway, which displays SMTP traffic between the Exchange server and the offsite server. However, it appears that some SMTP traffic is sent directly from the external network gateway IP to the destination domain IP.
Email that is sent directly to a domain using SPF authentication is being returned, since the SPF addresses do not match. If the destination domain is not using SPF, then the email is able to get through. Internal clients using Microsoft Outlook are not the problem.
If multiple email messages are sent, one will go through the gateway and the other will go through Exchange Server. Email sent to AOL.com, Yahoo.com, etc., tend to go out through the gateway instead of the Exchange server. Why is this happening?
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