Setting email headers like X-Auto-Response-Suppress can be very useful when enabling email notifications, as you may be flooded with auto-responses from absent employees etc.

Using a noreply email address can be an alternative in some cases, but not all (for us this is not an option).

I can imagine this is a minor fix. Either, if it's only this one header, put a boolean flag in the email configuration part which, if set, makes synapse add the header to all sent emails. Or, if multiple headers are interesting, allow some yaml-list-like structure where admins can set custom headers as needed.

Especially the first option is probably a very minor fix, but would have great impact on our use of synapse.