We have upgraded dovecot from 2.0.9 (centos6) to 2.2.36 (centos8).

If more than 1000 users log in, the following error will occur.

Nov 16 11:33:00 xxxxxxxx dovecot[1361]: imap-login: Error: master(imap): net_connect_unix(imap) failed: Resource temporarily unavailable - http://wiki2.dovecot.org/SocketUnavailable (client-pid=1362, client-id=129834, rip=10.10.10.10, created 551 msecs ago, received 0/4 bytes) Nov 16 11:30:26 xxxxxxxx dovecot[1361]: imap: Error: net_connect_unix(/var/run/dovecot/stats-writer) failed: Resource temporarily unavailable

I set the following because I thought it was a problem with the number of connections, but I get an error.

service imap-login { service_count=0 client_limit = $default_client_limit process_min_avail = 4 process_limit = $default_process_limit vsz_limit = 1G }

limits.conf

• soft nofile 4096
• hard nofile 4096

cat /proc/sys/net/core/somaxconn

4096

Is there any other tuning point? Where should I look?