If it was widely known that outlook was the legitimate suffix, there's no need to worry about this. If SAG-AFTRA had their own instance then any actor's account username associated with it would carry the suffix chosen by SAG-AFTRA.

TomHanks@sag-aftra.com for example.

TomHanks@lemmy.ml would be instantly recognizable as illegitimate.

This problem already exists in many different forms and is already managed well by the fact that celebrities' real usernames are well known and bullshit posts from accounts trying to fake them are easily caught just by looking at the user name. There are plenty of parody accounts on X with very similar username formats. Is that a major problem for X users? Not from what I've seen.