Any data transferred between two computers (a server being a computer...), must be transferred using a port and a protocol.
If you are using SMTP then the
default port should be
25. Bear in mind that you can change ports used on a computer system so your host may have changed this. You may wish to check with them. That is the
outgoing port, where data is sent
out from the server.
The
incoming port depends on the computer the person recieving the email on is using. If they have port forwarding setup it could be anything. But you can't really change this yourself if they are using Hotmail, Gmail, Yahoo! Mail etc as these are not on your server. If people are checking email actually on your server itself then you can choose which port your clients can use. By
default, this should be
587.
The PHP mail(); function, believe it or not, uses SMTP. So again, you're looking at double-checking with your host to see if anything's been changed. By as said, the
default is
25.
As a reference use:
http://php.net/mail.