monline

I did not know how exactly MTAs work, when they try to deliver a mail to the recipient. airwin helped me out: the MTA tries to look for an MX record for the given domain name. If found, then the given mail server is contacted. If not, then the host with the given domain name is contacted directly.

So eg. if an MTA is to deliver a letter to recipient@server.example.com, then first a DNS query is submitted for any MX records for the server.example.com domain name. If there's any MX record (eg. mail.example.com), then the mail is submitted to the host that is defined in the MX record. If there's no MX record, then the MTA tries to contact a mail server process on the host of the domain (server.example.com).

There can be more than one MX records for a domain name. Each MX record has a priority (with a numeric value) and the MTA will try to deliver the email in order of the priorities of these MX records.