You typed in your message and clicked on the Send button in your mailer. What happens next? How does email find its way to the final recipient?
Outgoing SMTP Server
Your Internet Service Provider has a special computer called mail server, which is responsible for collecting mail from their customers. When you click the send button, your computer connects to the mail server and transmits the message along with the list of recipients.
The message is transmitted with Simple Mail Transfer Protocol (SMTP). SMTP is a language that your mail program uses to speak to the mail server. SMTP lets your mail program specify the list of recipients and the text of the message.
The mail server that understands SMTP is frequently called Outgoing SMTP Server.
Destination SMTP Server
Your destination recipient also has an Internet Service Provider. They have a mail server too.
When your SMTP server decides that it's time to send your email, it connects to the destination mail server and transmits the message to it. They use SMTP to speak to each other. Therefore the destination mail server is frequently called Destination SMTP Server.
Destination SMTP server stores the email message until your recipient decides to check if some email arrived.
Incoming POP3 Server
When the recipient decides to check the email, his email program connects to the destination SMTP server and retrieves mail from the server. This time, they use Post Office Protocol Version 3 (POP3), which is a language used to receive mail. Therefore, this server is usually called Incoming POP3 Server.
Destination SMTP server for you is Incoming POP3 server for your recipient. When your recipient replies, the situation reverses - now your mail server becomes a Destination SMTP server from the viewpoint of the reply sender.
POP3 service is not necessary. There are other means to access mail on the destination mail server. For example, you can use Web browser to access Hotmail® or Yahoo Mail without downloading all the mail to your computer.
MX Records
One thing remains unclear. There are millions of SMTP servers all around the world. How your SMTP server knows where to send the message?
This information is stored in so called MX records. MX is not an abbreviation, it doesn't mean anything. The MX record shows which destination SMTP server must be used for the specific email address. For example, the MX record for aysoft@aysoft.com points to the server called slim.aysoft.com.
The database of MX records is maintained by a network called Domain Name Service (DNS). To get access to the MX records, you must have access to the DNS server and have a permission to retrieve MX records.
You can look up MX records for any e-mail address with special software, such as AY Spy.
Relay
As you might have noticed, there are two SMTP servers involved in sending each message. Your outbound SMTP server is working as a relay - it accepts your message and relays it to the other server.
When Internet just started, any SMTP server used to work as relays - the message used to be passed from server to server freely. As the email system got abused by spammers, fewer and fewer servers were working this way. Now all open relays are closed. The only server that will relay for you is the one given to you by your Internet Service Provider.
It is also possible to bypass all relays and send email directly to the destination server. For more details see our article about direct send.
