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May 05, 2010

Verizon FiOS disables SMTP to port 25

by Eric Smith — last modified May 06, 2010 08:30 PM
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This caused trouble for some of our clients, read below for the workarounds.

Today Verizon reconfigured residential FiOS in the Washington DC area to block TCP port 25 to non-Verizon servers. They claim this is for network security reasons, and I understand that this change will ultimately be made to the nationwide FiOS network.

SMTP interception: Not a new issue

For a long time we've recommended to our clients that they use port 1025 for sending email to our servers. Our servers listen to this port in addition to port 25. The main reason we recommend this is that many ISPs, in particular hotels, redirect port 25 to their own email servers. We definitely do not want our email to traverse these servers!

For those clients who have already switched to port 1025, today's action by Verizon was a non-event. For other clients (and indeed my own computers at home!) outbound email was broken until the clients were reconfigured.

Problems with Apple

Switching to port 1025 worked well for our clients using Thunderbird, but Apple Mail presented a unique problem. Old Apple Mail clients (2.1.4) worked, but for versions 3.1 and 4.2 the port setting would not survive a reboot. Fortunately Apple Mail tries multiple ports: 25, 465, and 587. So by adding port 465 to the list of ports we were already listening to, these Apple Mail clients started working without any reconfiguration.