MX Record Lookup & Mail Server Inspector
Verify Mail Exchange (MX) records, inspect server priorities, detect email security gateways (Proofpoint, Mimecast), and diagnose DNS misconfigurations in real time.
How Mail Exchange (MX) Priorities Work
MX records instruct other email servers on the internet where to deliver incoming mail. Understanding priority values is vital to preventing delivery failures:
Lowest Number = Highest Priority
Unlike ordinary lists, MX priority works backwards: a record with priority 1 or 10 is queried first before any record with priority 20 or 50.
Security Gateways (SEGs)
Enterprises configure Proofpoint, Mimecast, or Barracuda at the lowest priority (e.g., 10) to inspect all incoming traffic for spam, then relay clean emails to Microsoft 365 or Google.
Backup Relays (Failover)
Higher priority numbers represent backup mail exchangers. If the primary host is temporarily unresponsive, connecting MTAs hold mail and retry the backup server.
Critical RFC Scenarios Diagnosed by Our Engine
Most online tools simply dump raw DNS strings. NullBounce analyzes technical deliverability states according to IETF RFC standards:
RFC 5321: A-Record Implicit Fallback
FallbackIf an authoritative DNS zone specifies no MX records, section 5.1 of RFC 5321 mandates that MTAs attempt direct delivery to the IP address defined in the host's A or AAAA record. Our tool automatically tests this fallback before claiming a domain is dead.
RFC 7505: Explicit Null MX (No Mail)
RFC 7505Companies that never send or receive email on a specific domain (such as marketing subdomains or asset CDNs) publish a single record: 0 .. This explicitly tells sending MTAs to drop connection immediately without waiting for timeouts.
Split Delivery & Migration Leftovers
WarningWhen migrating from Google Workspace to Microsoft 365, administrators frequently leave old Google MX records active. This creates a split-brain state where sending MTAs round-robin deliveries between both providers, causing missing customer emails.
CNAME in MX Records (RFC 2181 Violation)
DNS ErrorRFC 2181 section 10.3 strictly prohibits pointing an MX record target to a CNAME alias. Doing so causes loopbacks and unpredictable bounce messages on strict corporate MTAs.
MX Record Check vs. Full Mailbox Verification
An MX lookup verifies the destination mail server exists. It does not verify whether an individual employee's mailbox exists on that server.
| Diagnostic Capability | This Free MX Tool | NullBounce Platform |
|---|---|---|
| Resolves Mail Exchange (MX) Hostnames & Priorities | ✓ Included | ✓ Included |
| Identifies Email Provider (Google, M365, Proofpoint) | ✓ Included | ✓ Included |
| Diagnoses RFC 5321 Fallback & RFC 7505 Null MX | ✓ Included | ✓ Included |
| Specific Mailbox Handshake (Does user@ exist?) | ✕ No | ✓ Included |
| Catch-All (Accept-All) Mail Server Detection | ✕ No | ✓ Included |
| Bulk Upload (CSV & TXT lists up to 100k rows) | ✕ No | ✓ Included |
MX Record Checker: Frequently Asked Questions
Answers to common questions about DNS mail routing and server configurations.
Why does my domain have multiple MX records with different priorities?
Major email providers (like Google Workspace) provide multiple hostnames (e.g., aspmx.l.google.com with priority 1, and alt1.aspmx... with priority 5) for high availability and load balancing. If the primary server experiences network congestion, sending servers automatically try the alternative hosts.
How long does it take for MX record changes to propagate?
DNS propagation depends entirely on your domain's TTL (Time-To-Live) value configured on your authoritative DNS provider. Typical TTL values range from 300 seconds (5 minutes) to 86,400 seconds (24 hours). If you recently modified records, some internet resolvers may cache old routes until the TTL expires.
What does the "Null MX (RFC 7505)" status mean?
A Null MX record is an official standard (RFC 7505) where a domain owner publishes 0 . to explicitly signal to all mail servers across the internet that the domain has no email service and will never receive email.
Does an active MX record guarantee an email won't bounce?
No. An MX record only confirms that the domain has an active server capable of handling email. It does not verify whether the specific mailbox (e.g. john.doe@company.com) is open, full, closed, or configured as an accept-all Catch-All. To check mailbox deliverability, use our Single Email Checker.
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