As someone who is constantly mailing items between friends, guildmates and alts on a secondary bnet account, it's really annoying.
Yes, I know them. Yes, they're in my friends list. Yes, they're a guildmate. If I didn't want to mail them something, I wouldn't.
Sure, it could be useful in the .1% chance that I misspell a name, but come on - make it a little smarter.