I actively endorse https://github.com/mailhog/MailHog https://github.com/mailhog/MailHog if you want something more Go-flavoured or operationalised. But it is a

I actively endorse https://github.com/mailhog/MailHog https://github.com/mailhog/MailHog if you want something more Go-flavoured or operationalised. But it is also "complete."