What Are MX Records and Why They Matter for Email
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MX records describe mail routing
An MX record tells a sending mail system which hosts handle incoming mail for a domain. Its preference number helps order delivery attempts: lower numbers are preferred. Several records can provide alternative destinations.
An MX lookup concerns the domain after the @ sign. It does not check whether the particular mailbox exists or whether a message will reach an inbox. Email Extractor finds addresses in supplied content; it does not perform DNS or delivery checks.
Inspect a domain without sending a message
On a computer with dig available, run:
dig example.org MX +noall +comments +answer
Replace example.org with the domain you intend to inspect. The comments include the DNS response status; the answer section shows returned records. Actual records can change, so compare the output with the current configuration rather than a screenshot of an old lookup.
On Windows, an available alternative is:
nslookup -type=MX example.org
A DNS lookup does not contact the named mailbox, though your resolver processes the queried domain. Do not put an entire address, password or private token in a domain lookup field.
Distinguish missing, null and failed answers
| Result | What it establishes |
|---|---|
| Ordinary MX records | Advertised mail-routing destinations exist |
Null MX, written as 0 . |
The domain explicitly advertises that it accepts no mail |
| Successful lookup with no MX answer | More investigation is needed; this alone is not proof of invalidity |
| NXDOMAIN | The queried DNS name does not exist at lookup time |
| Timeout or SERVFAIL | The lookup failed; do not classify it as a confirmed nonexistent domain |
SMTP's routing rules in RFC 5321 allow an implicit destination at the domain itself when no MX records exist, using its address records. That is why an empty MX answer alone should not trigger deletion of every address at that domain.
Null MX in RFC 7505 is a distinct, explicit no-mail signal. Do not confuse it with a failed query.
Review extracted lists carefully
Keep a complete source-bearing export before any cleanup. Record the domain, lookup date, result and reason for a decision. Do not parse a source-bearing CSV by splitting every line at commas or blindly cutting at @; quoted cells and additional columns need a proper parser.
A mail host may identify an infrastructure provider or security gateway, but does not establish company size or the final mailbox service. MX answers also cannot detect catch-all behavior, verify a person's identity or guarantee deliverability. For those distinctions, read email validation limits.
