MX Records
What is MX Records?
Mail Exchange (MX) DNS resource records (RFC 5321 / RFC 1035) that specify the mail server hostnames responsible for accepting incoming email on behalf of a domain, along with integer priority preferences (lower numbers indicate higher priority).
Why MX Records Matters in Production
Before any SMTP connection can occur, a sending MTA must resolve the recipient domain's MX records. If a domain has no MX records (and no fallback A record), a null MX (`0 .` per RFC 7505), or points to a parked/sinkhole server, every email sent to that domain will hard-bounce.
Engineering & Sender Best Practices
- • Always check DNS MX records before attempting SMTP delivery or allowing a B2B domain through your signup form.
- • Inspect the MX hostname provider (e.g., `aspmx.l.google.com` vs. `*.mail.protection.outlook.com` vs. Proofpoint) to understand gateway behavior.
- • Reject domains publishing an RFC 7505 Null MX record (`IN MX 0 .`), which explicitly declares the domain sends or receives no mail.
- • Publish at least two redundant MX records with appropriate priority values (`10`, `20`) for your own receiving domains.
Pre-Filtering Dead & Parked Domains Plus Gateway Fingerprint Routing
An enrichment pipeline processing 500,000 B2B company domains resolved DNS MX records as Stage 2 of verification. It immediately identified 19,500 domains with NXDOMAIN or RFC 7505 Null MX (`0 .`) records—avoiding 19,500 pointless SMTP connection timeouts—while classifying the remaining domains by gateway (Google Workspace, Microsoft 365 EOP, Proofpoint, Mimecast) to apply provider-aware verification rules.
Technical Implementation & Wire Example
; Production Domain Routing Mail to Google Workspace (Priority 1, 5, 10)
company.com. 3600 IN MX 1 aspmx.l.google.com.
company.com. 3600 IN MX 5 alt1.aspmx.l.google.com.
company.com. 3600 IN MX 10 alt3.aspmx.l.google.com.
; RFC 7505 Null MX Record (Domain Explicitly Accepts Zero Email)
noreply-assets.com. 3600 IN MX 0 .MailVeri resolves live MX records and identifies whether a domain uses Google Workspace, Microsoft 365, a custom MTA, or a dead/Null MX.
DNS Email Authentication & Routing Protocol Reference
Gmail, Yahoo & Microsoft 365 Bulk Sender Standards| Protocol | DNS Host & Type | RFC Spec | Operational Requirement & Failure Impact |
|---|---|---|---|
| MX | @ (IN MX) | RFC 5321 / 7505 | Routes inbound mail by priority; missing or Null MX (0 .) causes 100% hard bounce |
| SPF | @ / Return-Path (IN TXT) | RFC 7208 | Authorizes sending IPs/ESPs; must stay strictly ≤10 DNS lookups to avoid PermError |
| DKIM | selector._domainkey (TXT/CNAME) | RFC 6376 | 2048-bit cryptographic header signature; survives email forwarding intact |
| DMARC | _dmarc (IN TXT) | RFC 7489 | Enforces From-domain alignment (p=none, p=quarantine, p=reject) + RUA reports |
| ARC / BIMI | default._bimi (IN TXT) | RFC 8617 | Preserves auth across forwarders (ARC) and displays verified brand logos (BIMI) |
MX Records Explained: How DNS Mail Exchange Routing & Priorities Work →
Deep dive into MX record priority numbers, backup mail exchangers, RFC 7505 Null MX records, and email verification.
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Frequently Asked Questions About MX Records
How do MX priority numbers work when a domain has multiple MX records?
Lower integer values indicate higher priority. A sending MTA always attempts delivery to the lowest-numbered MX host first (e.g., priority `1` or `10`), and only falls back to higher-numbered MX records (e.g., `20` or `50`) if the primary mail server is unreachable.
What is an RFC 7505 Null MX record (`0 .`)?
A Null MX record (`yourdomain.com. IN MX 0 .`) is an explicit DNS declaration that a domain accepts zero incoming email. When a verifier or sending MTA sees a Null MX, it must immediately fail the address as INVALID without attempting an SMTP connection.
Can a domain receive email if it has an A record but no MX record?
Technically RFC 5321 permits falling back to a domain's A/AAAA record if no MX record exists, but in modern production almost no legitimate mailbox domain relies on implicit A-record fallback—99%+ of MX-less domains are web-only assets or parked domains that cannot receive mail.
Closely Related Terms
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