smtp - Questions about email reverse DNS check -

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our email server blocked, need find way solve this. outbound smtp ip, when send emails email server, if send them smtp relay on other server, ip receive side check reverse dns? email server ip or relay server ip?

the receiving mail server reverse dns check on mail server received message from. so, if mail server sends message through relaying mail server, sends message receiving mail server, receiving mail server reverse dns check on relaying mail server.

the reverse dns check works so: receiving mail server receives incoming connection 209.85.160.179. receiving mail server reverse dns lookup on ip, (in case) resolve mail-yk0-f179.google.com. confirm sending mail server in fact in google.com domain (and 209.85.160.179 not run spammer or spoofer setup ptr record 209.85.160.179 points mail-yk0-f179.google.com), receiving mail server forward dns lookup on mail-yk0-f179.google.com, make sure fqdn in fact resolve 209.85.160.179 (a spammer not able setup dns record fqdn in google.com's domain).

so, legitimate senders should have ptr record setup ip address mail server sends from, , fqdn ptr record points should resolve same ip. how spam filters identify mail home computers have been taken on bots or zombies sending spam - ips used home internet connections not pass reverse dns check.


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