Graham,
It would be really helpful if your questions were more specific and demonstrate that you've looked at the Library app & have a specific question about it.
If you right click on the OutgoingEmail BO in the Library app and click on 'Find references to' , you will see a list of places that OutgoingEmail BO is used.
Click on 'Reminder email' and you will see a rule that Creates an instance of OutgoingEmail (which is the log of emails in the Library app).
Like mentioned earlier, the this rule in the Library app is on for a single email at a time, so the rule is on a BO like Loan, etc.
In your case where you want to create a log for a mass email, the rule with CREATE will be in a Process rule, not a BO rule.
Tom