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Strictly speaking user@localserver is valid, but would fail test 2.

Same for user@{ai,ax,cf,dm,gp,gt,hr,io,kh,km,lk,mq,pa,tt,ua,va,ws,bar,college,host} and probably some other TLDs I didn't bothered to check.

True, but is it something you actually want to accept?

Yes. Or no.