It makes it more standard if we use 204 when the reply is indeed empty. So, it might make sense to use both 200 and 204 according to the response body being empty or not. Also, the header name might be better as X-Kolab-Mailfilter-Action.

In D5490#71580, @machniak wrote:

It makes it more standard if we use 204 when the reply is indeed empty. So, it might make sense to use both 200 and 204 according to the response body being empty or not. Also, the header name might be better as X-Kolab-Mailfilter-Action.

I changed the header name. I don't think returning a 204 is adding anything if we're not going to use it, and I prefer having the very clear 200 on all successful requests, and the header for the actual action to take.