You can run your program with a testing set of users, where all potential receivers of your mock messages are your own emails. Is it not enough?

It's what I do. However, I rarely integrate with email, and when I do it's for in-house email addresses.

That said, I was giving an example of why a mock might not be an appropriate solution, and not on the wider topic of what "enough" might be.

I can certainly think that testing the ability to send emails to, say, 5,000 different addresses, and check that they are received, might be easier with something like this system.