Mail queue very slow with 10 min delay

Hi All,

i'm having an issue with my plesk server, since few weeks my mail queue has become extremely slow.
Looking for this issue in the forum i've already tried to disable greylisting, uninstalling the known extensions that are currently having problems but the issue persist.
The only thing that i notice is that if there are some mail in the queue after about 10 minutes they are processed.

My plesk version is Plesk Obsidian Web Host Edition 18.0.68 Update #1, runnning on a CentOs 7...

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