SMTP (Simple Mail Transfer Protocol).

SMTP is a text based push protocol which is used to send email messages. It uses a store and forward mechanism and is best suited for email communications between servers that are constantly active. If you are an Exchange user then you are more than likely using SMTP mail. The only exception would be if you are collecting email remotely on your laptop when you would be using a POP collection method. (See POP mail). SBS users also have the advantage of a pop collection system whereby the Exchange server periodically requests mail from a mail server and stores the mail in the Exchange database.

SMTP uses port 25 by default for email transfer and this port must be open to allow email through any firewall that your company may be using.