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What can cause internet email sent by Microsoft Exchange Server to no longer be received?

Following is a list of items that can internet email sent by Microsoft Exchange Server to not be received:
  1. Your Internet Service Provider (ISP) has blocked all access to the internet because they have received numerous complaints regarding Unsolicited Bulk Email (UBE)  and/or other unacceptable traffic originating from a computer or computers on your network.  To resolve this issue, contact your Internet Service Provider and request that the block be removed.
     
  2. Your Internet Service Provider (ISP) has appled an SMTP Filter also called a Port 25 Block to your Internet Service at any time without notification or consent.  In doing so, all outbound email sent from Microsoft Exchange Server will no longer route.  After a period of time, you will very likely receive multiple Non-Delivery Reports (NDR) in Microsoft Windows Server Event Viewer and within Microsoft Outlook.  To resolve this issue, contact your Internet Service Provider and request that the block be removed.

  3. Your Internet Service Provider (ISP) has moved or taken offline their Domain Name Servers (DNS).  The Domain Name Servers can be defined in the TCP/IP Configuration on the Windows Server that houses Microsoft Exchange Server as well as within Windows Server - DNS Server to forward DNS Requests.  The latter being the proper configuration.  To resolve this issue, contact your Internet Service Provider and request the TCP/IP Addresses of the new Domain Name Servers or a timeline of when the existing Domain Name Servers will be back online.

  4. A change has been made to an internal Firewall which is now blocking SMTP Port 25.

  5. A change has been made to the Internet Router/Gateway which now prevents the Windows Server that houses Microsoft Exchange Server from accessing the Internet.

  6. The Exchange Information Store has been dismounted by Microsoft Exchange Server.  In this scenario, both internal and external email would no longer route.  Microsoft Outlook would be forced offline.

  7. The Simple Mail Transfer Protocol (SMTP) Service has Stopped.

  8. The DNS Server Service has Stopped.

  9. An unusually large email is being sent or received by Microsoft Exchange Server.

  10. Microsoft Exchange Server has been improperly configured and is being exploited using an Open Relay.

  11. Microsoft Exchange Server has been corrupted by a virus infection.

  12. The Forward and/or Reverse DNS Entry for your Mail Server has been removed from the Root Servers.

  13. The receiving Mail Server may have a Grey List or Black List in place that is preventing the email from being received.

  14. Your Mail Server has been added to a Real Time Blackhole List (RBL).  Determine the Hostname/IP Address of your Mail Server and then enter its IP Address into the RBL Lookup.
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