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

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http://serverfault.com – 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? (HowTos)