Hi, I am using DNN 6.2.3, and so far I have been able to accomplish this by doing the following.
1) As admin, go under site setting -> user account settings, and blank out the regular expression in the box "Email Address Validation"
2) For a user with multiple emails, have them put them in with a comma between them.
the newsletter seems to work fine (although the audit post will have a count of 1 per user, it does send to as many as you have defined in the email field.
Not sure yet if this is going to cause other problems, I just got this request today myself so I don't know yet if this mucks up other things, I'll find out :)
If it works, I'll try to come up with a proper regular expression so that it still forces people to put in something valid (either one or more valid email addresses), as the blanking it out is only meant to be temporary.
I hope this doesn't cause other problems, this would be a nice simple solution as I am in the same boat.
thks,
Phil