A quick Google for "outlook autoreply headers" gave me https://www.jitbit.com/maxblog/18-detecting-outlook-autoreplyout-of-office-emails-and-x-auto-response-sup

A quick Google for "outlook autoreply headers" gave me https://www.jitbit.com/maxblog/18-detecting-outlook-autoreplyout-of-office-emails-and-x-auto-response-suppress-header/ https://www.jitbit.com/maxblog/18-detecting-outlook-autorepl... -- a more comprehensive list of headers is available at https://github.com/jpmckinney/multi_mail/wiki/Detecting-autoresponders https://github.com/jpmckinney/multi_mail/wiki/Detecting-auto... Will some things filter through this list? Undoubtedly, but I would be surprised if it's a significant percentage. Extremely high-volume lists probably need some more specific care, to be sure.