aaa@domain.tld -> aaa
aaa+xxx@domain.tld -> aaa
bbb@domain.tld -> bbb
bbb+yyy@domain.tld -> bbb
ccc@domain.tld -> catchall
ccc+zzz@domain.tld -> catchall
aaa@domain.ttt -> (none)
aaa+bbb@domain.ttt -> (none)