SMTP, POP3 and IMAP are TCP/IP protocols used for mail delivery. Email clients use POP3 and IMAP protocol to retrieve email from the server (incoming mail) over TCP/IP connection. Outgoing mail for both POP and IMAP clients uses the SMTP. Main difference between POP3 and IMAP is,

• POP3 – Downloads email locally
• IMAP – Mail is stored on the mail server

SMTP

Simple Mail Transfer Protocol (SMTP) servers handle the sending of your e mail messages to the Internet. The SMTP server handles outgoing e mail, and is used in conjunction with a POP3 or IMAP incoming e mail server.

POP3

Post Office Protocol 3 (POP3) servers hold incoming e mail messages until you check your e mail, at which point they’re transferred to your computer. POP3 is the most common account type for personal e mail. Messages are typically deleted from the server when you check your e mail.

IMAP

Internet Message Access Protocol (IMAP) servers let you work with e mail messages without downloading them to your computer first. You can preview, delete, and organize messages directly on the e mail server, and copies are stored on the server until you choose to delete them. IMAP is commonly used for business e mail accounts.

Default Ports

• SMTP AUTH: Port 25 or 587 (some ISPs are blocking port 25)
• SMTP StartTLS Port 587
• SMTP SSL Port 465
• POP Port 110
• POP SSL Port 995
• IMAP Port 143
• IMAP SSL Port 993
• IMAP StartTLS Port 143

The following list of SMTP, POP3, and IMAP server should help you if you don’t know what mail server you should use for your mail account.