I have not had a chance to walk through this yet, but we are going to try to get on a call today 11 am est. I too can send test emails by going into the old flash interface > build > users.
I checked the release notes for 6.7 and there is no mention of this issue. I think the notes need to be updated, an advisory posted or the 6.7 release pulled from the download site until this is fixed. That's just my opinion.
This is what support said "We are working on getting an issue we have recently found with the older user accounts not being able to see the email address so, your rule is still firing but the LEM cannot read the email address any more as the SMTP settings changed around a little bit with the upgrade. However we do have a work around while dev continues to work on the resolution if you would like to do that. All we need to do is recreate the user accounts not getting email. This will put all their email settings in the right place and then they can receive the rule emails again."