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I would assume that this is for security purposes. I think it's great for those who may unleash a virus on their system that attacks addresses in saved emails.
I think you only get that behaviour with the addresses of people who you have in your Address Book/ Contacts.
I've never kept an address book. Don't ask me why. You just reminded me to dig through the Help files to see if this is a security feature to prevent the sending of email if a virus is unleased on a system. That would make sense to me.