Subj : Re: SMTP/Email Verification br0ken??
To   : paulie420
From : Zip
Date : Sat Jul 16 2022 09:43 am

Hello paulie420!

On 15 Jul 2022, paulie420 said the following...
 pa> I remember when I set it up, smtp2go asked for port 2525 to listen - but
 pa> I'm thinking that I pointed IT to my IP, and have since moved... so I'm

I just had a look at their setup pages, and the 2525 seems to be on their
end.

The reason for them offering that port number (and a bunch of other port
numbers as well) is that most ISPs block any outgoing traffic to port 25 (i.e., to an SMTP server somewhere).

This is to force customers to use the ISP's own SMTP servers to avoid being classified as a "spam network" (getting blocked in greylists and blacklists used by SMTP servers all over the world for anti-spam measures) in case your computer gets infected and starts sending out thousands of spam e-mails. That situation would prevent all of their customers from sending out e-mail, so needs to be avoided.

But, as you can see, restricting outgoing traffic based on port numbers isn't really helpful, as one can always use another port number. :-D

They offer a bunch of them -- among them 80, i.e. the web traffic port -- which the ISP simply can't block (as no web browsing would work then).

Of course, there is the possibility of the ISP monitoring each outgoing TCP connection from you, to auto-detect if you're talking SMTP on port 2525 or 80 and then close down that connection, but that would probably consume a lot of resources at their end, so I don't think many ISPs do that.

 pa> Thx as always, Zip.

You're very welcome! =)

Best regards
Zip

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