How mail uses MX
A message to user@example.com does not go to the website IP by default. The sender looks up MX for example.com, sorts by priority (lowest number first), and tries SMTP on each target. If every MX fails, some senders fall back to the A record. That fallback is a last resort, not a design. Domains that receive mail should publish explicit MX records.
Priority, backups, and providers
Google Workspace, Microsoft 365, Fastmail, and most ESPs give you one or more MX hostnames. Copy them exactly, including the trailing-dot convention your DNS UI expects. Two records at the same priority are a simple form of load sharing. A higher-priority (larger number) backup only helps if that host can actually queue mail for you.
After a provider move, stale MX is the usual reason mail still lands in a dead mailbox. Pair this check with SPF & DMARC so the new sender is authorized. Longer explainer: MX records explained.
Reading the result
The value column shows priority hostname. Confirm the hostname itself has A or AAAA records — an MX that points at a name with no address will not receive mail.