Re: Registration email query
I don’t know the ‘official’ answer, but I have had several users register a new name with an old email address (their username was spelled wrong or something) and they got their email and it works fine.
To test, I just re-registered myself with a new name and an old email address, and it was received. I am on a gmail account. I do know that MSN, AOL and Yahoo tend to be aggressive with their anti-spam, and there are a lot of false positives (I know this from other business mail I send.) user_email is not a UNIQUE field in the database either. (I am integrated with WP – not sure if it matters.)
I suspect it’s spam filtering catching the emails.