Ok - here's the deal.
In the version I have, when I ran the install/default.aspx and got the error about the field "NotificationType" missing in the database, I created it manually and install continued just fine (going from v1.9.3 to v1.9.5 rev 4221) - This is on my Snitz Migrated Forums. When the system dealt with all my users, it set the NotificationType to a value of 0. For some reason this causes major issue when YAF tries to decide whether to send notifications to topic posts. I found in my digging that Setting "No Notifications" on a user profile sets this value to "10" in the database for that user. I ran a SQL Query setting any user that had a NotificationType value of "0" to "10" and executed it and poof - no more of these errors are appearing. I had already checked all my imported users to locate any with blank email addresses (or invalid format email addresses) and none were found, so that wasn't the issue. It had to do with this NotificationType value being = to "0".
Don't know if this will help anyone else, but as far as the source goes, you might want to put a check in the install script that makes sure the value isn't set to 0 or it will cause problems when users attempt to post messages.
If you can't find it using the forum search, try my signature link -- searches this site using Google:
Google is my Friend