SMTP, which stands for Simple Mail Transfer Protocol, is the technology used to send out email messages. It goes hand in hand with POP (Post Office Protocol) that is, typically, employed for receiving emails from a web server to an email program or an email client.
When someone sends you an email, it’s transferred over the internet from one server to another using SMTP. When the message finally arrives at your email server, you can download it to your computer using an email client employing the Post Office Protocol.
The main function of text based SMTP protocol is to “push” emails – it cannot “pull” them from servers which is why you also need POP. The “outgoing” mail server protocol helps servers communicate with each other and facilitate the delivery of the email message.
SMTP functions in two ways. Firstly, it verifies the configuration of the computer from where the email is being sent and grants permission for the process. Secondly, it sends out the message and follows the successful delivery of the email. If the email cannot be delivered, it’s returned-to-sender or bounces back.
The enhanced version of SMTP called Extended SMPT or (ESMTP) is employed for sending images and other file attachments along with emails.
Since SMTP does not authenticate the sender, it has been exploited for sending spam. And because of this, most ISPs keep a close check on the number of outgoing email messages from each account. In fact, ISP generally do not allow account holders to send email from other SMTP address but their own and they do this by quickly blocking ports 25 (and/or 26). This leads to the most common complain – cannot send emails – from subscribers who configure a non ISP email account, such as those from their web sites or web based email providers, in a email client. The problem can usually be ironed out by contacting the ISP and asking for their SMPT outgoing server address, port number and authentication information.
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