Why so many apple mail port numbers?

I have several different email addresses, some of which all cause me duplication problems in apple mail. My solution is to set up some as IMAP and some as POP which for whatever reason solves the problem. The only place I get a road block is selecting a "Port" number in the advanced section of apple mail setup. Since the suggested numbers don't always work, I literally just type in different numbers until one does work.

What are these numbers? why do some work and some don't? Why don't the ones recommended (on email account instructions) work, yet other numbers I pick do?

Kate