Issue Mail senders are getting error sending mail to plesk mail

[ahmaddarwish]

Hi,

Many mail senders are getting below error when they are sending me emails to my domain hosted on plesk with mail active.
I've many other domains on same server all working well.

the error:

Deferred: 451 4.7.1 Service unavailable - try again later

 

[mow]

Many mail senders are getting below error when they are sending me emails to my domain hosted on plesk with mail active.

This is the postfix greylisting, which is a very efficient spam protection, with some drawbacks.
The error you see is a temporary error that postfix always throws on the first connect of a new peer.
As it is temporary (4xx), the sending server will try again later. When this happens, and it's after the timeout setting in postfix's config, the mail will get accepted on the second try.
Advantages:

• hit-and-run spammers don't retry

Drawbacks:

• mail delivery is deferred, obviously
• this does not work when the sender is not just a single host but a whole bunch of outgoing servers and a different one is chosen for each try, which will then get the temporary error again because postfix doesn't know that server yet, until you finally get one a second time.

Workarounds:

• put senders from which mail should be delivered immediately and senders that use a bunch of outgoing hosts into the whitelist