"Trumail is a free and open source email validation/verification system. It is available in three forms, the Golang client library verifier for use in your own Go projects, a public API endpoint (more info: https://trumail.io), and a public Docker image on DockerHub (see: https://hub.docker.com/r/sdwolfe32/trumail/).

NOTE: While we do offer a managed, enterprise level service to paying customers, it is highly recommended that you host the service yourself either using a Docker image or by forking and serving this project on your own instance. Please keep in mind, self-hosting Trumail requires bidirectional communication on port 25 which most residential ISPs restrict - AWS and Digitalocean both permit this sort of communication."

Last updated 3 years ago but maybe it's so simple it doesn't need updating. Be very handy to have, considering the comparable cost of these SaaS services.