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How I Solved Comcast's Block of my SMTP (Email) Port 25

I received an email from Comcast telling me that they are blocking my outgoing email:

Dear Comcast Customer:

ACTION REQUIRED: Comcast has determined that your computer(s) have been used to send unsolicited email ("spam"), which is generally an indicator of a virus. For your own protection and that of other Comcast customers, we have taken steps to prevent further transmission of spam from your computer(s).

Comcast did provide information on configuring my email client to get it to work. It's not too difficult if you are simply sending email, however my setup is not normal. The fix is basically to modify the client to send outgoing email using port 587 instead of the standard SMTP port 25. Also, the port must be configured with authentication (my Comcast ID and password).

For my situation, I run a home server to handle incoming and outgoing email so for me the fix for outgoing email is not to my client which talks to my email server (on port 25), but to fix the email server to send out using port 587 authenticated.

That was the easy part. However, my wife said she had not received any email for the last several days. I poked around and found out that incoming port 25 was also being blocked. I was not able to connect to my home server's port 25 from an external site (telnet 25). My wife has her own business domain for which email was directly sent to the home server ( This blockage did not affect my email to since the mail collects on that hosted site and my home email server periodically picks up the mail via POP3.

The solution is to use a different port than 25. However, there are several steps to get this to work.

  1. My (dynamic) DNS name server is handled by (free). They have a solution to redirect SMTP to a different port but costs $29/yr. I found a free redirection from a different company ( So the first step is to have Sitelutions redirect email to and with two "MX" records. 

  2. has this SMTP redirection free for a small amount of email (or $40 for a larger amount). It will also do filtering so I set up to only redirect email to debsrealty on port 2525. If you receive more email than the free account allows, combine step 1 and 2 and just go with the Sitelutions $29/yr solution or check with your own nameserver company.

  3. The 3rd step is to modify my router to accept incoming requests on port 2525 instead of 25. Also, the router configuration will route the requests to my home server on port 25 so I did not have to change the email server to listen on port 2525. I used to verify port 25 was indeed blocked and 2525 was now open.

After this was all done, email started coming in for Messages sent over the last few days often are retried by the various email transports and were received. It's certainly possible that some were lost.

The rollernet site keeps a log of email transactions that is useful in verifying functionality.

Print | posted on Wednesday, May 21, 2008 12:00 AM | Filed Under [ Personal ]


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