Julian Mehnle wrote:

Not only Courier has to discard the old message, but it has to accept a new one. Perhaps, the "resubmit path" can be optimized slightly? Anyway, it will remain a technique suitable for setups where messages are modified in a few cases only.

Filters that always add a header could overwrite a previously inserted header mask. We need a hook to pre-insert masks, then. Filters setting a result that maildrop filters can use, could set a variable in the control file. The latter implies maildrop should be able to retrieve such variables.

As an alternative mode, Courier could rewrite the message before invoking global filters. I.e., invoke global filters just before recipient-specific filters. It may be somewhat inefficient to rewrite a message if a filter is going to discard it. Thus it may still be better for some other filters to be called in one of the currently existing modes.

For yet another approach, a filter response could contain some modification commands, e.g. "add this header", "drop that attachment"...