Client change email port to 2525, now we can't send to them....How can we now?

Posted on 2007-04-11
Last Modified: 2010-03-06

We have a small client who uses an ISP for their email services.  One day they decided to change the mail port from 25 to 2525 to try to stop spam.  Well, since they've done this many folks (Us included) have been unable to send to them.  We just figured this out by chance and then called the client to verify.  Telnetting to port 25 of the server yields nothing now since their ISP is now blocking that.  Now, the question is how can I configure our Exchange 2003 server to send email to this particular domain over port 2525 without causing issues with the thousands of others we send to?


Question by:alchik
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LVL 31

Accepted Solution

LeeDerbyshire earned 125 total points
ID: 18890182
You can change the port number in Exchange System Manager by looking at the properties of the Default SMTP Virtual Server, and clicking the Advanced button.  That will allow you to send to this one client, but when everyone else is using port 25, you won't be able to send to anyone else.  The rest of the world assumes that SMTP servers listen on port 25 - they can't keep trying every port to discover the changed port number.  If they can't connect on port 25 they will just give up.
LVL 104

Expert Comment

ID: 18890230
The client needs to change their port back to 25. They cannot expect everyone who emails them to use another port. Asking Hotmail, AOL or any other major ISP to comply will be met with a polite no at best and a laugh at worst.

You cannot deal with spam by simply changing the port.


Author Comment

ID: 18892168
The client has no control over the ISP changing the port unfortunately.  After doing some googling it seems others are doing this as well.  I was sort of hoping there was a way of routing domain specific traffic to say, a specific connector/virtual server what would send out via 2525.  
They say there are receiving email from some people.  I can send to them from gmail and yahoo.
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LVL 104

Assisted Solution

Sembee earned 125 total points
ID: 18893389
I cannot believe that the ISP is expecting this to be a public facing port. As Lee has stated above - the entire SMTP system is based on the port 25. Like http is port 80, https is 443 and POP3 is 110.
This sounds like either a massive cock up by the ISP or someone has taken the private port (for client to ISP communication) and is treating it as the public facing port.


Author Comment

ID: 18897854
I agree with you Sembee.  I've emailed their ISP support group directly (funny, their mail server works just fine...) to see if I can get an answer.  The client has tried to get an answer from them but the ISP claims it's everyone else's fault.  It's bizarre but seems it's not an isolated incident. offers MailHop to get around such things.  Odd....
LVL 104

Expert Comment

ID: 18897917
The service that is offering is a private service. You send your email to them and then they send it on to the internet using the conventional ports. I suspect this is a similar service.


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