re 993 and 143 - I think the former is the 'secure' version of the latter.
The 143 port is used for IMAP.
IMAP is a method of accessing your mail where you can pull just the 'headers' (from/ date/ to/ subject) onto your mail application, and can choose to pull the 'body' later (ideal if you were being charged lots of cash to download and didn't want to download some photos, but wanted to see an important e-mail offering a lucrative contract).
With IMAP, you could have laptop and desktop and a PC at the office and all could see the mail messages... 'pulling' a message to your mail application doesn't wipe it off the mail server, just copies it onto your machine.
There are options to delete the mail off the mail server, of course, but IMAP is ideal if you are using a service with web access too, as you can view mail from an internet cafe (or just using a browser when away in a hotel or somewhere), and leaves the mail for download into as many computers as needed (second machine may be used to keep backup copies, for example)
POP is a different method, where your mail client (usually) downloads the items off the mail server. Once those are downloaded, they cannot be 'pulled' by another mail client on the same or another PC. It's possible to 'leave mail on server' but managing POP mail is a bit more awkward if you want multiple computers to have the mail.
(Apologies, not meaning "PC" above to mean Windows PC, just the proper meaning, personal computer, so MacBook Pro, iMac, etc, under same umbrella!)