> логи ошибок приложите где эта ошибка фигурирует mail.log:
Nov 24 09:27:16 mail dovecot: auth: Debug: auth client connected (pid=2108)
Nov 24 09:27:16 mail dovecot: auth: Debug: client in: AUTH#0111#011PLAIN#011service=imap#011session=mvvgbkMl4gAKFMVT#011lip=10.20.197.96#011rip=10.20.197.83#011lport=143#011rport=61666
Nov 24 09:27:16 mail dovecot: auth: Debug: client passdb out: CONT#0111
Nov 24 09:27:16 mail dovecot: auth: Debug: client in: CONT<hidden>
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): bind search: base=ou=people,dc=company,dc=local filter=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=test3))
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): result: uniqueIdentifier=test3; uniqueIdentifier unused
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): result: uniqueIdentifier=test3
Nov 24 09:27:16 mail dovecot: auth: Debug: client passdb out: OK#0111#011user=test3
Nov 24 09:27:16 mail dovecot: auth: Debug: master in: REQUEST#0113251109889#0112108#0111#0114f237f9b4dad41a7a5dc431f6ff5b1ed#011session_pid=2109#011request_auth_token
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): user search: base=ou=people,dc=company,dc=local scope=subtree filter=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=test3)) fields=mailHomeDirectory,mailUidNumber,mailGidNumber,mailStorageDirectory
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): result: mailGidNumber=5000 mailHomeDirectory=/srv/vmail/company.ru/test3 mailStorageDirectory=maildir:/srv/vmail/company.ru/test3/Maildir mailUidNumber=5000; mailStorageDirectory,mailUidNumber,mailHomeDirectory,mailGidNumber unused
Nov 24 09:27:16 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<mvvgbkMl4gAKFMVT>): result: mailGidNumber=5000 mailHomeDirectory=/srv/vmail/company.ru/test3 mailStorageDirectory=maildir:/srv/vmail/company.ru/test3/Maildir mailUidNumber=5000
Nov 24 09:27:16 mail dovecot: auth: Debug: master userdb out: USER#0113251109889#011test3#011home=/srv/vmail/company.ru/test3#011uid=5000#011gid=5000#011mail=maildir:/srv/vmail/company.ru/test3/Maildir#011auth_token=01f88233af6feda21347f5918357888494a62fa9
Nov 24 09:27:16 mail dovecot: imap-login: Login: user=<test3>, method=PLAIN, rip=10.20.197.83, lip=10.20.197.96, mpid=2109, session=<mvvgbkMl4gAKFMVT>
Nov 24 09:35:31 mail dovecot: auth: Debug: Loading modules from directory: /usr/lib/dovecot/modules/auth
Nov 24 09:35:31 mail dovecot: auth: Debug: Loading modules from directory: /usr/lib/dovecot/modules/auth
Nov 24 09:35:31 mail dovecot: auth: Debug: Module loaded: /usr/lib/dovecot/modules/auth/libauthdb_ldap.so
Nov 24 09:35:31 mail dovecot: auth: Debug: Read auth token secret from /var/run/dovecot/auth-token-secret.dat
Nov 24 09:35:31 mail dovecot: auth: Debug: auth client connected (pid=2202)
Nov 24 09:35:31 mail dovecot: auth: Debug: client in: AUTH#0111#011PLAIN#011service=imap#011session=KtVejEMlXAAKFMVT#011lip=10.20.197.96#011rip=10.20.197.83#011lport=143#011rport=61788
Nov 24 09:35:31 mail dovecot: auth: Debug: client passdb out: CONT#0111
Nov 24 09:35:31 mail dovecot: auth: Debug: client in: CONT<hidden>
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): bind search: base=ou=people,dc=company,dc=local filter=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=test3))
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): result: uniqueIdentifier=test3; uniqueIdentifier unused
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): result: uniqueIdentifier=test3
Nov 24 09:35:31 mail dovecot: auth: Debug: client passdb out: OK#0111#011user=test3
Nov 24 09:35:31 mail dovecot: auth: Debug: master in: REQUEST#0113156738049#0112202#0111#011a84cc318cd90a2b125e16d9f9360897d#011session_pid=2205#011request_auth_token
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): user search: base=ou=people,dc=company,dc=local scope=subtree filter=(&(objectClass=PostfixBookMailAccount)(uniqueIdentifier=test3)) fields=mailHomeDirectory,mailUidNumber,mailGidNumber,mailStorageDirectory
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): result: mailGidNumber=5000 mailHomeDirectory=/srv/vmail/company.ru/test3 mailStorageDirectory=maildir:/srv/vmail/company.ru/test3/Maildir mailUidNumber=5000; mailStorageDirectory,mailUidNumber,mailHomeDirectory,mailGidNumber unused
Nov 24 09:35:31 mail dovecot: auth: Debug: ldap(test3,10.20.197.83,<KtVejEMlXAAKFMVT>): result: mailGidNumber=5000 mailHomeDirectory=/srv/vmail/company.ru/test3 mailStorageDirectory=maildir:/srv/vmail/company.ru/test3/Maildir mailUidNumber=5000
Nov 24 09:35:31 mail dovecot: auth: Debug: master userdb out: USER#0113156738049#011test3#011home=/srv/vmail/company.ru/test3#011uid=5000#011gid=5000#011mail=maildir:/srv/vmail/company.ru/test3/Maildir#011auth_token=7949497056b40527e5f853e1dcdee9486d7ed8d4
Nov 24 09:35:31 mail dovecot: imap-login: Login: user=<test3>, method=PLAIN, rip=10.20.197.83, lip=10.20.197.96, mpid=2205, session=<KtVejEMlXAAKFMVT>
Ошибку я сам не увидел в логах. Ошибка появляется в почтовом клиенте (thunderbird)