Question SMTP error Service unavailable

[intradesign]

Server operating system version

AlmaLinux 8.6

Plesk version and microupdate number

Obsidian 18.0.46

Hi,
I am using Spam Experts Spam Protection with Plesk. I have the following problem: Incoming mails are often not delivered to the recipients' mailboxes on the Plesk server, but end up in the queue on the Spam Protection portal. The following error message appears:

SMTP error from remote mail server after pipelined sending data block: 451 4.7.1 Service unavailable - try again later

(where "remote mail server" is the Plesk server).

The mails in the queue are delivered in further attempts within 30 to 40 minutes. So no mails will be lost. However, there are time-critical mails (such as two-factor authentication codes) that must arrive immediately.

Does anyone have a similar problem and is there a solution?

I have temporarily activated plesk premium mail. OS is AlmaLinux 8.6.

Do you need more info?

Thanks and best regards

 

[Kaspar]

Do you have Greylisting enabled in Plesk?

 

[intradesign]

Yes. Should it be disabled?
Anyway: Do I need server wide spam protection in combination with spamexperts? I don’t think so…

 

[Kaspar]

Disabling Greylisting is advised if you're using external spam filtering. Which I think would solve your initial issue. In addition you could disable server wide spam protection as it's redundant.