Another alternative that I've been using for ~5 years is MailHog https://github.com/mailhog/MailHog

For Windows, there's also the lovely Papercut: https://github.com/ChangemakerStudios/Papercut-SMTP

The relay mode for Maildev looks interesting though, but also vaguely terrifying as an accidental way to send test emails out to people!

If you are using Windows there is SMTP4Dev

https://chocolatey.org/packages/smtp4dev

Smtp4dev is also now available on Mac and Linux and Docker images are available:

https://github.com/rnwood/smtp4dev

I’ve been using mailcatcher for a long time. https://mailcatcher.me/

Same!