3.1.3. MX records

MX records — contain the addresses of mail servers that receive incoming mail sent to the domain's mailboxes.

MX record priorities allow multiple mail servers to be used for a single domain and determine the order in which attempts to use them should be made. This increases the likelihood of mail delivery and allows incoming mail to be received by several physical servers.

When sending an email, the sender's mail server checks the recipient domain's MX records. From these records, it obtains a list of mail servers and their priorities. Priorities are assigned with numbers — the smaller the number, the higher the priority. The sender's server then goes through the recipient's server list in descending order of priority (from the smallest number to the largest) until it successfully delivers the email or determines that delivery is impossible.

For example, the recipient's domain has two MX records: mx10.example.com with a priority of 10 and mx20.example.com with a priority of 20. Initially, sending attempts will always be made to the server from the first record, as it has a higher priority (lower number). If this server is unavailable, then sending attempts will begin to the server from the second record.