Lotus Notes Unique User Names

We’ve just moved away from our divisional messaging system and are running our own Domino R5 environment. I’m seeing a weird thing going on with addressing mail. If I have a user named John Doe and I send him an e-mail addressed to john@[domain].com he will receive it, even if he is the only John in the address book and his last name is not addressed. If I add a dummy user with the same first name – the mail gets trapped (In my opinion, as it should!)with the error that the user name is not unique. This wouldn’t be a problem but I have a few users (with unique first names) who receive spam when it should get trapped.
So my question is…is this by design? or is there something that can be done to correct this? Domino shouldn’t deliver mail if the name is not complete.