How to prevent plaintext authentication via IMAP/POP3 and SMTP in Postfix on Plesk server?

Connect to the server via SSH

Enable PCI compliance to Dovecot service:

# plesk sbin pci_compliance_resolver --enable dovecot

Enable PCI compliance to Postfix service:

# plesk sbin pci_compliance_resolver --enable postfix

Edit the file /etc/postfix/main.cf adding the following line:

CONFIG_TEXT: smtpd_tls_auth_only=yes

 Open the smtpd.conf file in a text editor (in this example, we are using the vi editor) and remove "PLAIN" and "LOGIN" from mech_list:

• on CentOS/RHEL-based distributions

  # cat /usr/lib64/sasl2/smtpd.conf
  
  pwcheck_method: auxprop saslauthd
  auxprop_plugin: plesk
  saslauthd_path: /var/spool/postfix/private/plesk_saslauthd
  mech_list: DIGEST-MD5 CRAM-MD5
  sql_engine: intentionally disabled
  log_level: 4

• on Debian/Ubuntu-based distributions

  # cat /etc/postfix/sasl/smtpd.conf
  
  pwcheck_method: auxprop saslauthd
  auxprop_plugin: plesk
  saslauthd_path: /private/plesk_saslauthd
  mech_list: DIGEST-MD5 CRAM-MD5
  sql_engine: intentionally disabled
  log_level: 4

Restart mail services to apply the changes:

# service saslauthd restart && service postfix restart && service dovecot restart