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What is MX record?
A DNS record that names the servers that receive mail for a domain, in priority order.
How it works in practice
Lower priority numbers are tried first. An MX must point to a hostname with an address record, never directly to an IP or to a CNAME. A sending domain should still have valid MX records, because replies and bounces need somewhere to arrive.
The common mistake
Leaving a sending-only domain without MX, so replies and bounces vanish and the domain looks throwaway.
Check it yourself
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