This might need some tweaking as it currently opens mpv in the
foreground preventing me from viewing other mails while it plays.
folder-hook youtube push '<limit>=(=U|=F)<enter>'
# delete other mails
folder-hook youtube push '<delete-pattern>!~(~U|~F)<enter>'
+
+# open video in mpv
+folder-hook * bind index,pager m mail
+folder-hook youtube 'macro index,pager m "<pipe-message>grep -E \"https?://www.youtube.com/watch\\?v=\" | xargs mpv<enter>"'