What Is SMTP and How Can I Use It?

What is SMTP?

It is short for Simple Mail Transfer Protocol – a protocol for sending email messages between servers. Most email systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an email client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify the POP or IMAP server and the SMTP server when you configure your email application.

It is the easiest and fastest way to get email from one place to another. Once you transmit your mail to the SMTP server, it will be sending it out. If some failures occur, it will be trying again and again. Since the real SMTP server is constantly connected to the Internet, it can try resending its failures at any time. As a result, you do not have to worry about failed recipients. If you send direct, you will have to resend to the temporarily failed recipients several times to make sure all the addresses are covered.

What does this mean as far as my email is concerned?

Let us look at the way this all works. Email is composed and read in an email client. This is the part of the process that is most familiar to you. If you want to read mail you click Check Mail, New Mail, or something similar and new messages show up in your Inbox. To send mail you click Compose or Write and the client gives you a compose window where you write your message. When you are done you click Send and the message gets sent.

The “Client” part of the term email client refers to the fact that this software requires the help of a Server to do its job. The client takes care of making the mail useful to you but the servers are the ones that actually move the mail around. Think of the client as your desk (or wherever you store and compose paper based mail) and the servers as the postal service.

First, your client must locate a Simple Mail Transfer Protocol server to send mail through. This will generally be provided by an Internet Service Provider (ISP) or the host for the domain the email address belongs to. The client knows how to find this server because the server’s name or IP address was entered in the client’s configuration for the sending address.

Passing the message to the SMTP server is roughly equivalent to walking a letter down to the corner post box and dropping it in. As far as you are concerned it is sent.  Similarly, after passing the message to the sending SMTP server, your email client will say something like “Message sent successfully” because at that point its job is done.

One major difference between the mailing a letter and sending an email message is that an email message can have multiple recipients. If you want to send many copies of a letter using snail mail you have to make many copies and address a lot of envelopes. With email you just use one envelope with a lot of addresses.

After the SMTP server accepts the message from your client it works its way through all of the recipients (everyone with a “To:” or a “Cc:” or a “Bcc:”) and tries to deliver the message to each one. This is where the Domain Naming System (DNS) and specifically Mail Exchanger (MX) Records come in. The sending SMTP server has to consult the DNS MX records for each recipient’s domain to find out where to send the message. This is similar to your local post office sorting your letters for delivery to your local post office.

Nearly all SMTP relay servers on the Internet restrict access to their own users. In most cases, SMTP relay servers allow access based on the IP address of the email client. On the other hand, some SMTP relay servers require authentication with a user name and password. No matter how authentication is done, it is easy to use an SMTP relay server as long as permission has been granted.

Did You Know? Many Internet service providers use DNSBLs (DNS-based Blocking Lists) to disallow mail from open relays. Once a mail server is detected or reported that allows third parties to send mail through them, they will be added to one or more such lists, and other email servers using those lists will reject any mail coming from those sites. The relay need not actually be used for sending SPAM to be blacklisted.

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