What this check looks at
MX (Mail Exchange) records tell the world where to deliver mail for a domain. Using the envelope sender's domain, JoltMx checks that:
- MX records exist (or there's an A-record fallback).
- The domain doesn't publish a null MX (RFC 7505
0 ., meaning "I receive no mail"). - Each MX host is a hostname, not an IP literal (which RFC 5321 forbids).
- The MX hosts actually resolve to an address.
Why it matters
Replies and bounces need somewhere to go. A domain that can't receive them frustrates your recipients and looks suspicious to filters - many receivers penalise or reject mail from domains that can't receive mail in return. A dead or missing MX also means delivery-status notifications about your own mail vanish.
How the diagnostic grades it
| Result | Severity | What it means |
|---|---|---|
| MX records resolve | Pass | The domain has MX hosts and they resolve to addresses. |
| Domain cannot receive mail | Critical | Neither MX nor A records - bounces and replies are undeliverable. |
| MX host does not resolve | Critical | An MX hostname has no A record - mail to the domain fails outright. |
| Declares it receives no mail (null MX) | Warning | An 0 . record. Give the domain a working MX if it sends mail. |
| No MX - implicit A fallback | Warning | Works but fragile and penalised by some filters. Publish an explicit MX. |
| MX points at an IP address | Warning | IP literals aren't allowed - point the MX at a hostname. |
| No envelope domain to check | Info | The message had no usable sender domain (e.g. a bounce). |
How to fix it
- Publish at least one MX record pointing at a hostname that has an A/AAAA record.
- Remove dead MX entries whose hostnames no longer resolve.
- Only publish a null MX on domains that genuinely never send or receive mail.
Setting records up for the first time? See Add and verify your domain.