I, in a state of way too tiredness, sent mail to my buddy's "shipname@handle" as opposed to "character@handle". So I'm curious what will happen.
Your friend will get the mail. Mail is account-wide, not character specific. You can send it to anyrandomtext@handle and it will work. In the same way, you can see all mail sent to you when checking mail from any of your characters. No matter what anyone tells you, the way it actually works is how I just explained. Try it out yourself if you want to see.