I have a Dovecot/Postfix/MariaDB on a Centos, just have a user ask me:

I recently upgraded my Thunderbird email client and have experienced problems since. It appears that when Tbird polls for new messages it gets held up waiting for a response from the server I'm using POP port 995. Any ideas as to why I'm having a problem ?

how to investigate such issue ?

CentOS release 6.10 (Final) # dovecot --version 2.3.11.3 (502c39af9) load average on server 0.05/0.01 user log shows like:

Oct 12 10:08:50 pop3-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=17371, TLS, session=<E83xW8rqDPkxw3EW> Oct 12 10:09:34 pop3(user@tld.com.au)<17371><E83xW8rqDPkxw3EW>: Info: Disconnected: Logged out top=0/0, retr=0/0, del=0/114194, size=10616009472 Oct 12 10:14:36 imap-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=17460, TLS, session=<ftiLcMrqR/gxw3EW> Oct 12 10:14:37 imap(user@tld.com.au)<17460><ftiLcMrqR/gxw3EW>: Info: Logged out in=152 out=69724 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=1 body_bytes=68628 Oct 12 10:28:27 pop3-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=17714, TLS, session=<+WsXosrqK/gxw3EW> Oct 12 10:29:16 pop3(user@tld.com.au)<17714><+WsXosrqK/gxw3EW>: Info: Disconnected: Logged out top=0/0, retr=1/76174, del=0/114195, size=10616085600 Oct 12 10:32:19 imap-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=17817, TLS, session=<bZftr8rqq/gxw3EW> Oct 12 10:32:20 imap(user@tld.com.au)<17817><bZftr8rqq/gxw3EW>: Info: Logged out in=153 out=135592 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=1 body_bytes=134503 Oct 12 10:45:58 imap-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=18081, TLS, session=<U3674Mrqzfgxw3EW> Oct 12 10:45:59 imap(user@tld.com.au)<18081><U3674Mrqzfgxw3EW>: Info: Logged out in=152 out=24602 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=1 body_bytes=23514 Oct 12 10:48:49 pop3-login: Info: Login: user=<user@tld.com.au>, method=PLAIN, rip=49.111.222.333, lip=103.xxx.yyy.zzz, mpid=18135, TLS, session=<y1/r6srqtPsxw3EW> Oct 12 10:49:53 pop3(user@tld.com.au)<18135><y1/r6srqtPsxw3EW>: Info: Disconnected: Logged out top=0/0, retr=2/166190, del=0/114197, size=10616251743 Oct 12 10:50:42 imap(user@tld.com.au)<3274><cr10yqTqifgxw3EW>: Info: Logged out in=192516 out=5957294 deleted=0 expunged=0 trashed=0 hdr_count=263 hdr_bytes=358656 body_count=126 body_bytes=5110403 Oct 12 10:50:42 imap(user@tld.com.au)<3271><hLl0yqTqh/gxw3EW>: Info: Logged out in=78324 out=58677 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0

Voytek