My sieve mail filter does not work

If your filter rules do not work, there may be several reasons.
If the result of the filters you set up is different than you expected, it is usually because the filters were configured incorrectly or because several filters influence each other. Unfortunately, we cannot provide support for your individual filter configuration. Deactivate all filters and reactivate them one by one to find out where errors occur.

Integrated / external mailboxes

If you retrieve e-mails from external mailboxes that you have integrated into mailbox, the Sieve filters or filter rules are not applied to these mailboxes. This applies to your own filters as well as to our spam and virus filters.
The background is that the Sieve filters only apply to e-mails that are "normally" sent to the mailbox of your mailbox address via SMTP. However, e-mails from integrated mailboxes are not transferred to your mailbox, but are only displayed in your account - similar to processing your e-mails via IMAP with your local mail program.
To apply your filters to these emails as well, you would either have to forward them from your external account to your mailbox account or have them fetched from there with a POP3 retrieval.