Mail Clients are no longer able to use encrypted password authentication type after update to Plesk Obsidian 18.0.73

Symptoms

• Plesk was recently updated to versions 18.0.73, Dovecot updated to version 2.4:

  # plesk -v
  Product version: Plesk Obsidian 18.0.73.0 
  OS version: AlmaLinux 9.6 x86_64 
  Build date: 2025/10/02 10:00

  # dovecot --version
  2.4.1-4 (7d8c0e5759)

• Mail Clients, such as Thunderbird, receive the following error when attempting to log into a mailbox using "Encrypted Password" option:
  
  

• When checking IMAP/SMTP server capabilities, no MD5 auth methods are advertised:

  # openssl s_client -connect 203.0.113.2:993
  . 
  OK [CAPABILITY IMAP4rev1 LOGIN-REFERRALS ID ENABLE IDLE SASL-IR LITERAL+ AUTH=PLAIN AUTH=LOGIN] Dovecot ready. 
  # openssl s_client -connect 203.1.113.2:465 -crlf 
  250-AUTH PLAIN LOGIN

• Non-descriptive lines are being logged in /var/log/maillog, even with Dovecot Debug enabled:

  # dovecot[24549]: auth: Debug: Loading modules from directory: /usr/libexec/dovecot/modules/auth
  dovecot[24549]: auth: Debug: Loading modules from directory: /usr/libexec/dovecot/modules/auth 
  dovecot[24549]: auth: Debug: Module loaded: /usr/libexec/dovecot/modules/auth/libauthdb_plesk.so dovecot[24549]: auth: Debug: Read auth token secret from /var/run/dovecot/auth-token-secret.dat 
  dovecot[24549]: auth: Warning: Weak password schemes are allowed 
  dovecot[24549]: auth: Debug: conn unix:login (pid=24580,uid=991) [1]: Server accepted connection (fd=18) 
  dovecot[24549]: auth: Debug: conn unix:login (pid=24580,uid=991) [1]: auth client connected (pid=24580) 
  dovecot[24549]: imap-login: Login aborted: Connection closed (no auth attempts in 0 secs (no_auth_attempts): user=<>, rip=203.0.113.3, lip=203.0.113.2, TLS, session=<94tUujpAK88KQzTS> auth: Debug: conn unix:login (pid=24580,uid=991) [1]: Disconnected: Connection closed (fd=18)

Cause

Product issue:

• PPPM-15147: "After updating to Plesk Obsidian 18.0.73, Thunderbird could not send or receive emails when “Encrypted Password” was selected in the email client settings"

Fixed in:

• Plesk Obsidian 18.0.73 Update 2 6 October 2025

Resolution

Update Plesk to the latest version:

How to install Plesk updates