I'm scoping out a project that will involve sending outbound email messages, and I'm a little confused about why one would use a MassEmailMessage instead of multiple SingleEmailMessages.
If you look at the documentation, Singles have far more options available when it comes object types that can be used in the setWhatId methods, especially since they can accept Custom objects. Mass emails have very limited options for the setWhatIds.
Single and Mass emails have separate limits, but they both have the same limit of 1,000 external email addresses per day.
And when it comes to using the Messaging.sendEmail(), you have pass in an array of messages anyway, so why not send in an array of many Singles instead of an array of one Mass?
So what am I missing? What's the advantage of using a MassEmailMessage for sending out a batch of emails when the SingleEmailMessages have more options?