The email service I use has a library that allows me to set the Host as a constructor parameter, so I have an EmailHost environment configuration value that I change. It's localhost for dev/testing and set to the correct host for production. My test code then listens for the API calls from that library, uses a different library to parse the email, and then I can verify the content.

No code changes between dev, test, and production, but I added an environment variable to represent the e-mail host so that I can change where e-mails are sent as needed.

I dislike mocking, so I took the time to figure that out and set it up. If mocking works for you, I likely wouldn't go out of my way to change things to get rid of it.