How mail servers communicate

ICE_MAN

The problem is not as simple as the protocol. The problem is how the 21cn server finds the hotmail server. We know that hotmail does not support smtp at all, nor does it provide the corresponding ip and port for us to connect, but why the 21cn server can send to the hotmail server mail?

心系部落

First, the server knows the other party's IP and other information through DNS domain name interpretation.
2. The mail server must apply for a domain name and resolve it to send and receive mail through the Internet.
3. The communication between mail servers has been verified (DNS), so you can send messages to each other freely

The following is an example of the mail routing process
The SMTP server routes email based on the domain name of the intended recipient in the DNS service. SMTP server is based on MX records in DNS
To route emails, MX records register domain names and related SMTP relay hosts, and emails belonging to this domain should be sent to this host.

If the SMTP server mail.abc.com receives a letter to be sent to:

a. Sendmail requests DNS to give the CNAME record of host sh.abc.com, if yes, if CNAME goes to shmail.abc.com, then again
Request the CNAME record of shmail.abc.com until there is no
b. Suppose CNAME is sent to shmail.abc.com, and then sendmail requests the DNS of @abc.com domain to give the MX record of shmail.abc.com,
shmail MX 5 shmail.abc.com
10 shmail2.abc.com
c. Sendmail finally requests DNS to give the A record of shmail.abc.com, namely the IP address, if the return value is 1.2.3.4
d. Sendmail connects with 1.2.3.4 and sends this letter to the SMTP background program of this server