I would go with module option even if it's a simple one function module. Keeping a generic function that can eventually be used by multiple classes in a module makes much more sense than defining it inside a class.
If Rails 4.3 is of your concern then you can simply replace your include MySafeEmailModule
with whatever Rails 4.3 would include this function in, as compared to find and replace all calls to ReportMailer.daily_emails
.