What is domain existence verification?

An email address is made of two parts: the local part (the username before the @) and the domain (the part after it). Domain existence verification checks that second part. Specifically, it checks whether the domain actually exists in DNS and whether it's set up to receive email.

The check looks for MX records. An MX record (Mail Exchange record) is a DNS entry that tells other mail servers where to deliver messages for a domain. If a domain has no MX records, there's nowhere to send email, and the address is undeliverable. If the domain doesn't resolve in DNS at all, it doesn't exist and the address is invalid.

This sounds basic, but it catches a surprising number of problems in real lists:

• Typos in the domain: captain@harborpot.net instead of captain@harborpost.net. The local part looks fine. The domain doesn't exist.
• Expired domains: A company shuts down or forgets to renew their domain. All addresses at that domain become undeliverable.
• Garbage domains: Fake signups using domains like test.com or fakeinbox.xyz that were never set up for real email.

Domain existence verification is one step in a full address validation workflow. It happens after syntax checking (does this look like a valid email format?) and before the mailbox-level check (does this specific inbox exist on this domain?). Skipping it means some addresses that would immediately hard bounce on send slip through as apparently "valid."

If you're running validation before a large send and your tool is flagging a lot of domain-level failures, that's either a sign that your list has aged significantly or that some of your acquisition sources aren't doing basic quality checks. Both are worth investigating. Regular cleaning keeps domain-failure rates from building up over time.