This seems like a flaw in the tryToCreateAccount semantics that we should fix.  I think we should have tryToCreateAccount create a MailAccount instance and pass it in.  For now, for simplicity, it will just be a dead MailAccount that does not update/generate events like one that lives in a slice, but I think it would be a good improvement.

Longer term, we probably just want the MailAPI instance to always have a built-in live/updating list of accounts and arrange the event sequences so that we know the account is present on success in our built-in slice and we can pull it out of there.  The deal is just that you'd need to open an additional/explicit slice of accounts if you wanted to set .element/.data on them for UI binding purposes.