What is SMTP?
SMTP or Simple Mail Transfer Protocol is a TCP/IP used to send and receive email. The majority of email systems use SMTP to send messages from one server to another. Messages can then be retrieved using an email client using an email download protocol such as IMAP or POP3.
SMTP also delivers email from a mail client to a mail server, thus the need to specify both an incoming and outgoing mail server when setting your email client.
SMTP uses ports 25 or 587 as some ISPs will block port 25.