See also Mailslurper [0] and Mailcatcher [1] if you'd like to self-host an email catch-all service.
Mailslurper appears to be abandoned and seems to be missing deterministic builds that go module support provides.
Mailcatcher has been around for a while, with my first discovering it being used in a legacy project's test suite back in 2014.
Both could use some better SEO since it's nearly impossible to find unless you know exactly the right keywords to use.