FTP is synchronous but SMTP and POP are both synchronous and asynchronous
SMTP port number is 25 for pushing the mail
By default, POP3 protocol works on two ports : Port 110
this is the default POP3 non-encrypted port
Port 995
this is the port you need to use if you want to connect using POP3 securely
MIME (Multipurpose Internet Mail Extensions)

At first a mail Client comes in, they fetch information from user (their mails)
This mail goes to MTA (Mail Transfer Agent)
Now from MTA it goes to another mail server (another MTA) where the mail client collects the mail and then the user gets to view