would need a special module on the api, generating configoutput from the core. under review, but more developers for design/implementation needed.

since the guide remains now in the wiki, and it is from the discussion and analysis definitely a core addon, feel free to catch in and enhance this guide or create a better script/daemon yourself.

https://wiki.icinga.org/display/howtos/Add+mailfilter+to+acknowledge+notification+mails+by+replying

i consider this resolved, either with official repos or addon repos.