MailHog is pretty great. In response to your concern, by default it runs on a non-standard port, so you need to configure your apps to point at it (port 1025 by default). Even then, you can configure it to act as a MITM, where the webmail interface lets you "release" messages you actually want to send out: https://github.com/mailhog/MailHog/blob/master/docs/CONFIG.m...
Personally, my local SMTP actually sends email, because I DO want legitimate email (mutt, cron) though, so I'd be scared to mix up two local smtp instances.