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A client have following set up -

  1. Dedicated CentOS 6.x server running Postfix + Dovecot 2.0.9 in some data center.

  2. 5-6 Windows 10 client computers with latest Thunderbird (IMAP)

  3. Some Windows 7 client computers with latest Thunderbird (IMAP)

Windows 7 computers have no any problem accessing the email server or problems are very rare.

All Windows 10 computers have problems accessing the emails. Sometimes it works, sometimes - it doesn't.

Often error is something like "can not save email in sent/draft folder. Try again?".

Issue appear on both SSL (port 993) and plain text connections (port 143) .

Today, during one of these errors, I was able to install telnet client and was able to connect to the IMAP server, however I did not try to login there.

I also noticed, that on the server netstat reporting 100+ connections from office IP to the mail server.

On this specific Windows 10 client, I noticed windows' netstat reports about 10 connections to the server as well.

On the server's log file I do not see anything strange.

More or less something like this:

May 23 13:09:45 mail2 dovecot: imap-login: Login: user=<dans@xxxxx.net>, method=PLAIN, rip=1.1.1.1, lip=2.2.2.2, mpid=18391, TLS
May 23 13:10:03 mail2 dovecot: imap(ticket@xxxxx.net): Disconnected: Disconnected in IDLE bytes=183968/34225
May 23 13:09:28 mail2 dovecot: imap(venceslava@xxxxx.net): Connection closed bytes=397/1848
May 23 13:09:30 mail2 dovecot: imap(ticket@xxxxx.net): Disconnected: Disconnected in IDLE bytes=214309/1256882
May 23 13:09:30 mail2 dovecot: imap-login: Login: user=<ticket@xxxxx.net>, method=PLAIN, rip=1.1.1.1, lip=2.2.2.2, mpid=18376
May 23 13:09:33 mail2 dovecot: imap-login: Login: user=<ticket@xxxxx.net>, method=PLAIN, rip=1.1.1.1, lip=2.2.2.2, mpid=18379, TLS
May 23 13:09:37 mail2 dovecot: imap(agency@xxxxx.net): Disconnected: Logged out bytes=38530/851

The only additional settings in dovecot.conf file are:

mail_max_userip_connections = 1500

service imap {
    process_limit = 2048
}

I can provide full config but is nothing unusual.

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