Precedence: bulk email header is non standard and discouraged
Feature #2984 requested adding a "Precedence: bulk" header to email notifications to prevent MS Exchange server from triggering out-of-office auto-responder.
However, RFC 2076 says this header is "Non-standard controversial, discouraged". It is also not supported by Amazon SES service (emails with the "Precedence" header are rejected).
The post referred to in issue #2984 suggests another way of preventing OOF autoresponder: use the "X-Auto-Response-Suppress:OOF" header. I believe this should be used instead of the Precedence header.