In light of the above, how would you feel about a patch which makes it possible to turn filtering and autoresponders on/off by using a checkbox per mailserver? We could set them to 'enabled' by default so the current behaviour is not broken. Then we could hide or show the options for filtering and autoresponders depending on these checkboxes.
This seems to be a good option in my opinion, the checkbox to enable autoresponder and filtering can be added to e.g. System > server config > mail if it shall be configurable per mail server or in System > Interface config > Mail if it shall be configurable for all servers that belong to this ispconfig installation. I guess the second option might be better. The first option might confuse users as we can not hide the filtering and autoresponder tabs until they saved the new mailbox as we dont know the target mailserver before the address is saved. If more users get interested in cyrus support in future, then we can still develop cyrus support then.