- Plesk for Linux
The emails are forwarded to another mailbox but the forwarding is disabled in Plesk > Domains > example.com > Mail Addresses > firstname.lastname@example.org > the Forwarding tab
CONFIG_TEXT: sieve: email@example.com: redirect action: forwarded to firstname.lastname@example.org
There is an additional filter in webmail configured.
Log in to webmail
Go to Settings > Filters
Find the filter that can be responsible for the redirect. For example:
Remove the filter by clicking the Delete button