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?".

MailCatcher (Ruby, https://mailcatcher.me/) is a good and open solution for this, MailHog (Go) is open as well, even simpler to set up (no worries about Ruby versions) and has never caused me any trouble.