dovecot - Dec 2019 - Problem with big number of pop3-login [1 pre-login]

Dear all,
I have recently migrated from my old Debian 8 (jessie) system to a new VM with
fresh?Debian 10 (buster) installation.
Previous dovecot?version was?2.2.13 and now I have?2.3.4.1.
Dovecot was setup from Debian?packages.
Mostly, it is working, but one problem is just?frustrating me.
The number of some pop3-login?processes are getting higher and higher until they
reach my login_max_processes_count?option which is?equal 1000.
From ps aux it?looks like this:
?
dovecot ?55067 ?0.0 ?0.0 ? 8192 ?6340 ? ? ? ? ?S ? ?00:09 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?55344 ?0.0 ?0.0 ? 8192 ?6388 ? ? ? ? ?S ? ?00:35 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?55351 ?0.0 ?0.0 ? 8192 ?6268 ? ? ? ? ?S ? ?00:35 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?55354 ?0.0 ?0.0 ? 8192 ?6388 ? ? ? ? ?S ? ?00:35 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?55594 ?0.0 ?0.0 ? 8192 ?6280 ? ? ? ? ?S ? ?00:36 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?56822 ?0.0 ?0.0 ? 8192 ?6300 ? ? ? ? ?S ? ?00:36 ? 0:00
dovecot/pop3-login [1 pre-login]
dovecot ?56996 ?0.0 ?0.0 ? 8192 ?6268 ? ? ? ? ?S ? ?00:36 ? 0:00
dovecot/pop3-login [1 pre-login]
?
?
Once it reaches the login_max_processes_count?value my users start to complain
to no new mail is arriving.
Restart resolves the?problem just?for some time, until it will come to a limit
next time (sometimes it take 1 day, sometimes 1 hour).
In my old system I had nothing like that, but I guess it could be done something
wrong during migration.
I don?t know if it is significant but instead of real users I deal with?only one
third-party application which utilizeses 5 simultaneous tcp?connections to check
mailboxes via SSL\TLS.
My dovecot is not public, and there is nothing?else besides these 5 connections.
Currently,?in a config I have this:
?
service pop3-login {
? process_min_avail = 4
? process_limit = 4
? client_limit = 100
? service_count = 100
?
Please friends give me a way how to handle the?situation.
Might it be?a bug?
?
========Regards,
Greg Semenov
?
?
?
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