What Is an MX Record?
TL;DR
An MX record is a DNS record naming a mail server that accepts mail for a domain, together with a preference number. A sending server collects the MX records for the domain, tries the lowest preference value first, and moves to the next one when a host does not answer.
How do MX priority values work?
The number is a preference and lower means preferred, so a host at 10 is tried before a host at 20. The values are relative and carry no other meaning: 1 and 2 behave exactly like 10 and 20.
Equal values are a load-sharing instruction. A sender is expected to choose at random among hosts sharing a preference, which is how providers spread inbound mail across several hosts without a load balancer in front.
What happens when a domain has no MX record?
The sender falls back to the address records for the domain itself, treating the A or AAAA record as an implicit mail exchanger. Mail then goes to whatever is at that address, which is usually a web server with no way to accept it.
That fallback is why deleting an MX record does not stop inbound mail cleanly. A domain that should never receive mail publishes a null MX instead, a single record with preference 0 and a target of one dot, which tells senders to reject immediately.
Does the MX record affect the mail you send?
Not directly. MX governs inbound routing while outbound trust rests on SPF, DKIM and DMARC, so changing an MX record does not change how your campaigns are treated.
It matters in two indirect ways. Receivers do look at whether a sending domain can accept mail at all, and your own replies and bounces only reach you when the inbound side works, so a domain used for outreach needs both halves configured.
Frequently asked questions
Yes, and most do. Several records with different preference values give a delivery order and a fallback, while several sharing one value spread the load. The case to avoid is records pointing at two different mail systems, because mail then arrives in two places.