Many ISP's is block port 25, how do I choose an alternative port?

Xeoncross asked:

I am building an application that will be acting as a combined MUA/MTA on different networks. However, many of the networks are with ISP’s that block port 25 for SMTP. Therefore I would also like to open up a secondary port so that some of the installs can communicate on that if port 25 is closed.

How do I choose a second port? I know some people use port 26 or port 2525. What is the correct way to choose a port that won’t interfere with existing software?

My answer:

If you’re just sending email on behalf of an end-user, who originates the email within your application, use authenticated SMTP to port 587.

You can send mail to some servers via the SMTPS port 465, if the remote MTA is SSL/TLS-enabled and listening on that port, but don’t count on it.

Otherwise, advise the user that they need a business-class ISP.

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