For two reasons. Firstly, you want people to have the correct email address - because they can see the domain, and if it's wrong, you'll cause brand confusion.

For two reasons. Firstly, you want people to have the correct email address - because they can see the domain, and if it's wrong, you'll cause brand confusion. Secondly, you have to setup all these special mechanisms to correct peoples' typos, permutations of which can be numerous. Think about the compounding complexity of your system as you cater to the very few cases of people making an error that can otherwise be alerted to them to be fixed. ...not to mention the potential for security vulnerabilities. Will your anti-spam filters work on all these alternate domains? Will you setup SPK DKIM etc for them all to prevent spoofing as well? Will you have website forwards... ...the list of things to think about goes on. For what? To cater to that one idiot and his fat fingers? Just tell him he fucked up.