Mailabruf mit STARTTLS und Port 143 teilweise nicht mehr möglich

[Andreas Wass - Glas Gasperlmair]

Hallo Liste,

Immer mehr Thunderbird-Clients (Version 91.10.0) können über Port 143 
und STARTTLS keine E-Mails mehr abrufen.
Irgendwie komisch, dass es nicht alle Clients betrifft, sondern täglich 
1 -2 dazukommen.

Derzeitige Lösung:
Servereinstellungen im Thunderbird auf SSL/TLS und Port 993 umstellen.

Im mail.log gibt es folgende Einträge dazu:
Jun 20 13:02:17 mail dovecot: imap-login: Disconnected (no auth attempts 
in 100 secs): user=<>, rip=192.168.106.93, TLS handshaking: Connection 
closed, session=<56qKCd/hyMLAqGpd>
Jun 20 13:02:21 mail dovecot: imap-login: Disconnected: Shutting down 
(no auth attempts in 19 secs): user=<>, rip=192.168.106.93, TLS 
handshaking, session=<ddfJCd/hzcLAqGpd>

Anbei noch meine Dovecot-Config: (doveconf -n)
# 2.3.13 (89f716dc2): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.13 (cdd19fe3)
# OS: Linux 5.10.0-8-amd64 x86_64 Debian 11.3 ext4
auth_mechanisms = plain login
default_client_limit = 8192
default_process_limit = 2048
default_vsz_limit = 4 G
log_timestamp = "%Y-%m-%d %H:%M:%S "
login_log_format_elements = user=<%u> method=%m rip=%r mpid=%e %c %k 
session=<%{session}>
mail_location = maildir:/var/vmail/%d/%n/Maildir
mail_plugins = fts fts_lucene zlib
maildir_stat_dirs = yes
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date index ihave duplicate mime foreverypart extracttext
mbox_write_locks = fcntl
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     auto = subscribe
     special_use = \Drafts
   }
   mailbox Junk {
     auto = subscribe
     special_use = \Junk
   }
   mailbox Sent {
     auto = subscribe
     special_use = \Sent
   }
   mailbox Trash {
     auto = subscribe
     special_use = \Trash
   }
   prefix =
   separator = /
}
passdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
plugin {
   fts = lucene
   fts_autoindex = yes
   fts_lucene = whitespace_chars=@.
   sieve = ~/.dovecot.sieve
   sieve_before = /var/vmail/sieve/global/spam-global.sieve
   sieve_dir = ~/sieve
   zlib_save = gz
   zlib_save_level = 6
}
protocols = imap lmtp sieve
service auth {
   unix_listener /var/spool/postfix/private/auth {
     mode = 0666
   }
}
service imap-login {
   process_min_avail = 32
   service_count = 0
}
service lmtp {
   inet_listener lmtp {
     address = 127.0.0.1
     port = 24
   }
}
service managesieve-login {
   inet_listener sieve {
     port = 4190
   }
}
ssl_cert = </etc/letsencrypt/live/mail1.glasgasperlmair.at/fullchain.pem
ssl_cipher_list = ALL:!LOW:!SSLv2:!EXP:!aNULL
ssl_key = # hidden, use -P to show it
userdb {
   args = /etc/dovecot/dovecot-sql.conf.ext
   driver = sql
}
protocol lmtp {
   mail_plugins = fts fts_lucene zlib sieve
}
protocol imap {
   mail_plugins = fts fts_lucene zlib imap_zlib
}

Vielen Dank im Voraus für eure Unterstützung,

Andi