For networks, a port means an endpoint to a logical connection. The port number identifies what type of port it is. Here are the default email ports for: -

POP3 - port 110

IMAP - port 143

SMTP - port 25

HTTP - port 80

Secure SMTP (SSMTP) - port 465

Secure IMAP (IMAP4-SSL) - port 585

IMAP4 over SSL (IMAPS) - port 993

Secure POP3 (SSL-POP) - port 995