MX
The MX record controls the delivery of the mail to your mail server.
Imagine sending an email to someone outside your company. Your mail server must therefore send the mail outwards and in the direction of the recipient. Where does your mail server get the information about which mail server it needs to talk to? The answer is in the MX record of the recipient domain.
What is an MX record? How does e-mail transport between mail servers work?
For example, if you send an e-mail to info@secumail.de, your mail server will first ask for the MX of the domain secumail.de . The answer is:
secumail.de. IN MX 20 mx-a.secumail.de. secumail.de. IN MX 20 mx-b.secumail.de.
Now the mail server determines the IP addresses behind it and knows who it can talk to.
mx-a.secumail.de. IN A 212.11.225.17 mx-a.secumail.de. IN A 212.11.225.2 mx-b.secumail.de. IN A 212.11.224.11 mx-b.secumail.de. IN A 212.11.224.4
The sender can choose any address from this list and deliver the mail to this mail server via SMTP.
Multiple MX records or multiple IP addresses per mail server increase availability and distribute the load across multiple servers.
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