Naturally, our servers support Sieve, which offers you the option of filtering or redirecting e-mails into other IMAP folders directly on the server. You can make all the necessary settings in mailbox.org Office under Settings => Mail => Filter Rules. Or you can use the Sieve manager options in your local e-mail program. You can use your program to set up the Sieve rules on our server using the standard Sieve ports 2000 and 4190.

Can Sieve filters also be applied to embedded / external mailboxes?

No, unfortunately not. You can read more about the reasons in the corresponding FAQ article.