This happens because Dovecot limits the maximum IMAP connections per IP to just 10.  This may be fine for a single client side IP but if an entire office or multiple users are behind one IP or a single heavy user is active then you will get bizarre errors in your e-mail clients such as "Password Incorrect" or similar in Thunderbird.  It won't be obvious on the client side as to what the problem is and they will probably just think the server is misconfigured.

 dovecot: imap-login: Maximum number of connections from user+IP exceeded (mail_max_userip_connections=10): user=<user@domain.com>, method=PLAIN, rip=10.15.12.5, lip=10.15.12.59, TLS
Jul 10 14:16:21 mailserverdovecot: imap-login: Warning: SSL alert: where=0x4008, ret=256: warning close notify [10.15.12.5]


add this to dovecot.conf:


  mail_max_userip_connections = 50

Restart dovecot and after that you should be good to go.

Note that some users say you have to put the above in an imap stanza like this (but it was not necessary for me):

protocol imap {
  mail_max_userip_connections = 50
}
 

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