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

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

Григорий Семенов
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