Emails are delivered to Inbox folder and moved to another folder

Symptoms

• Emails are delivered to Inbox folder and moved to another folder (e.g. to Spam or Junk folder) after several seconds.

• The following row can be found in /var/log/maillog:

  CONFIG_TEXT: dovecot: service=lda, user=john.doe@example.com, ip=[]. msgid=0739521947f6ca364cc20ae0f9299ac8@example.com: saved mail to INBOX

• No logs regarding moving the delivered email to another folder appear in /var/log/maillog even with enabled debug.

• The email physically is located in non-Inbox folder on the file system:

  # ls -l /var/qmail/mailnames/example.com/john.doe/Maildir/.Spam/cur/
  
  -rw------- 1 popuser popuser 826 Mar 15 01:21 1521051700.M284184P8137.hostname.com,S=826,W=846:2,S

• No filter rules are configured in webmail (Roundcube > Settings > Filters or Horde > Mail > Filters).

• SpamAssassin is disabled in Plesk > Tools & Settings > Spam Filter.

Cause

Filter rules are configured on some mail client side (e.g. Outlook, Thunderbird, etc.) which are moving emails from Inbox folder to another one.

Resolution

Disable/remove filter rules on the mail client side (e.g. Outlook, Thunderbird, etc.).