Mail delivery in outline.
WHAT HAPPENS WHEN YOU SEND
Your client connects to your mail server and submits the message Your server looks up the recipient domain's MX records It connects to the recipient's mail server and delivers The recipient's server files it, or rejects it
WHAT MX RECORDS DO
Name the servers accepting mail for a domain, with priorities.
WHAT HAPPENS ON REJECTION
Your server receives an error and generates a bounce.
WHAT A TEMPORARY FAILURE CAUSES
Retrying, over a period, before giving up.
That is why some messages arrive late rather than failing.
WHAT AFFECTS ACCEPTANCE
Authentication records Sending reputation Content, to a lesser extent
WHY DELIVERY IS NOT GUARANTEED
Acceptance is the receiving server's decision.
WHAT YOU CONTROL
Your records, your server's behaviour, and what you send.
WHAT TO CHECK WHEN MAIL FAILS
The bounce text, which usually states the reason precisely.
