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There are times when Exchange Server administrators have to pinpoint why email can be sent successfully to people outside the network but external email does not arrive. There are a number of conditions that can cause Exchange mail flow problems. To address the issue, examine every component that is involved in receiving email. Here are a few of the more common possibilities.
First, check any filtering applications that you use. A malfunctioning antivirus or antispam filter can cause issues with inbound Exchange mail flow. Try temporarily disabling filters to see if email begins to flow.
Also check your system services. Any Exchange Server related services -- including services that are used for third-party add-ins such as filtering software -- configured to start automatically should be running. Remember to check all of your Exchange servers for these services.
The next thing to check is your domain name system (DNS) server. Be sure that the DNS server's MX record is pointing to the correct IP address. While you are at it, make sure that the MX record reflects the correct fully qualified domain name.
Mail flow issues can also result from problems with an edge server. If your organization uses an edge transport server, make sure that it is running, that all Exchange-related services are functional, and that the server can communicate with the outside world and backend Exchange servers.
Finally, be sure to consider any non-Exchange Server related changes that have been made to your network recently. Something as simple as a change to a firewall rule or a new group policy setting can interfere with Exchange Server. There have even been documented situations in which Exchange mail flow was disrupted as a result of an incorrect change to a wide area network load balancer.
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