SMTP In A Nutshell
What is SMTP?
You need SMTP service if you wish to send email from email programs like Microsoft Outlook, Thunderbird, Mac Mail or mobile devices like the iPhone or Blackberry. If you only want to send email from WebMail, then you do not need SMTP services.
If you imagine that the sending of an email message is analogous to the sending of a letter, then computers are "post offices", and the "Simple Mail Transport Protocol" (SMTP) is the "procedure" by which a post office receives a letter or sends it off to another post office closer to the ultimate recipient.
SMTP is used by any program that sends an email message to deliver that message to the nearest "post office" for relaying to its destination.
For most people, there are really only two significant ways to send email - via a web-based interface or via an "email program" such as Microsoft Outlook or Thunderbird. In the former case, the use of SMTP is taken care of by the web server running the web-based email package, and as a result you don't have to worry or think about SMTP at all.
In the latter case, where you are using a program on your personal computer (or mobile device) to send email, you have to specify a "post office" for these programs to connect to such that they can send messages. These "post offices" are known as "SMTP servers".