MX lookup — a domain's MX records
Who receives mail for the domain and whether those servers are configured properly. Filters reject mail from domains that cannot accept replies.
Why MX matters for deliverability
Many filters (Rspamd, SpamAssassin, Microsoft) check whether the sender domain accepts mail at all. No MX, or an MX pointing to a non-existent host, is a spam signal and the cause of reject_unknown_sender_domain errors. Null MX (0 .) says explicitly that the domain does not receive — use it only for domains you never send from.
What we check
Every MX host is resolved to IPs and checked for PTR and FCrDNS (the PTR name pointing back to the IP). A server without FCrDNS is rejected by some receivers, and for a self-hosted MTA that is both MX and outbound server it is a prerequisite for delivering to Gmail.
FAQ
My MX points to an ESP — is that a problem?
No, as long as the host resolves and accepts mail. Common: Google (aspmx.l.google.com), Microsoft (*.mail.protection.outlook.com), OVH, IONOS.