Hvorfor får man meddelelsen "Sender must IP resolve", når man sender til mig.

Enkelte afsendere får deres mail til mig retur, med fejlmeddelelsen "Sender must IP resolve".

Som en del af spamfiltreringen på vores (og de fleste andre udbyderes) mailservere checkes det at den afsendende mailserver har det der kaldes en Reverse DNS.

Det betyder f.eks. at man kan slå vores mailserver op som mail.gullestrupnet.dk og få den IP-adresse 89.249.3.115 men også den modsatte vej, at slå IP-adressen op og få mailserverens navn.

Manglende Reverse DNS betyder som regel, at mail'en ikke kommer fra en "rigtig" mailserver, men f.eks. fra en spammer der afsender mails vha. sin engen (eller en hacket) computer.

Reglene findes i RFC1912 sektion 2.1:

2.1 Inconsistent, Missing, or Bad Data
Every Internet-reachable host should have a name.  The consequences of this are becoming more and more obvious.  Many services available on the Internet will not talk to you if you aren't correctly registered in the DNS.

Make sure your PTR and A records match.  For every IP address, there should be a matching PTR record in the in-addr.arpa domain.  If a host is multi-homed, (more than one IP address) make sure that all IP addresses have a corresponding PTR record (not just the first one). Failure to have matching PTR and A records can cause loss of Internet services similar to not being registered in the DNS at all.  Also, PTR records must point back to a valid A record, not a alias defined by a CNAME.  It is highly recommended that you use some software which automates this checking, or generate your DNS data from a database which automatically creates consistent data.

Det er administratoren på afsenderens mailserver, der skal lave ændringen.