Trumail - open source email validation / verification
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- https://trumail.io/
- https://github.com/trumail/trumail
- https://hub.docker.com/search?q=trumail&type=image
"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.