As per comments on https://gerrit.wikimedia.org/r/#/c/176366/ :
9:56 PM <tonythomas> hoo: We will have to change our current index.
9:57 PM <tonythomas> https://github.com/wikimedia/mediawiki-extensions-BounceHandler/blob/master/sql/create_index.sql
9:57 PM <hoo> Either that, or create a new one
9:57 PM <hoo> I think changing should be fine
9:57 PM <hoo> selectivity should be high enough, so that br_user_email doesn't need to be in there.