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See which servers receive a domain's email, in the order senders try them, with each server's IPv4 and IPv6 addresses and reverse DNS.
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Each mail server name is resolved to its addresses, and each address is checked for a PTR record and whether that PTR name resolves back to the same address (FCrDNS). It also flags the mistakes that make mail bounce: an MX that names an IP address, an MX host with no address, and an MX that is an alias.
This reads public DNS only and does not connect to the servers. To check that the domain's mail is authenticated, use the Email Security Check; to see every record type, the DNS Lookup.
Senders try the lowest number first. Hosts with the same number share the load; a higher number is a backup that is used when the lower ones do not answer.
Forward-confirmed reverse DNS: the IP address has a PTR record, and the name in that PTR resolves back to the same IP. Receivers check it on the servers that SEND mail to them, so it matters for an MX host only if the same server also sends.
A single MX record of 0 . (RFC 7505). It states that the domain does not accept email, so senders fail immediately instead of retrying for days. It is correct for domains that never receive mail.
Senders fall back to the domain's own A or AAAA address (the implicit MX rule in RFC 5321). That is rarely what anyone intends, so publish MX records, or a null MX if the domain should not receive mail.
No. It reads public DNS only, through a public resolver over DNS-over-HTTPS. It does not open an SMTP connection, so it does not test whether a server accepts mail or offers STARTTLS.
Add your domain to CloudSpex to be told when its MX records or email authentication change.
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