You can just use MailHog for free, it works pretty good. https://github.com/mailhog/MailHog
I'm a fan of MailHog. It's simple for the whole team to setup and the optional chaos monkey button is a great way to make sure your email system can tolerate periodic failures properly.
It works locally and is really simple to setup for shared staging/prerelease environments, handles high volumes really well and the websocket auto-update system is great.
Personally, I'm a fan of using Mailhog for local and Mailtrap.io for shared environments because in my experience, non-technical testers have a better experience with Mailtrap and it's simple tie ins to review the look of emails on different platforms/clients.
Basically, Mailhog for local sending, does it send, can it handle failure, etc and Mailtrap.io for "does it look right?".