Crackers do this with leaked password dumps already. They use the dumps as their core dictionary and start modifying the entries to broaden their attack.
What is useful though is: Using
myEmail+registeredOnSite@provider.tld
The sites can of course filter this, but you can easier sort incoming mail and possibly see when a site had a leak.
The "+registeredOnSite" part is ignored during email routing and you get the mail at
myEmail@provider.tld.
(Similar to Gmail, where it ignores anything appended with a . to your regular address name)
This works, because one of the biggest implementations used as MTA does this. So you need to
check if it actually works with your provider!
Too bad lots of sites limit the password lengths or won't allow spaces (underscores work ok though).
I just don't understand why they would limit the length so much. Often only 16, 20 or 32 characters are allowed. 32 are borderline ok...