How to prevent cleartext / plaintext authentication via IMAP/POP3 and SMTP in Postfix on Plesk server?
Note: If you don't have root access to the Plesk server via SSH, contact your service provider regarding this issue.
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Plesk with Dovecot + Postfix
Note: If Qmail is used, it is recommended to switch to Postfix. Forcing secure connection over SMTP in Qmail requires patching which is not supplied by Plesk.