What systems handle each step (client → MTA → DNS → receiving MTA)?

The sending client composes the message and passes it to a sending MTA such as Postfix, Exim, or a cloud provider’s gateway. That MTA queries DNS to fetch the domain’s MX record. DNS resolvers translate those requests into IP addresses.

The sending MTA then connects to the receiving MTA listed in the MX record using SMTP. Once connected, the receiving MTA performs checks including SPF, DKIM, DMARC, volume reputation, and spam analysis before deciding where the message should land.