Avoid users creating “split head” accounts. Moving a couple of posts over to a users “real” account, and getting them to use that is no big deal, but without a user merge it becomes really tedious to move more than about 10-15 posts.
You’re fortunate if you haven’t got many accounts with emails that no longer exist. If I turn email summaries on we will get blacklisted by various ISPs pretty quickly (about 40K accounts, about 1000 of them are active on the new platform).