Email verification is no secret. All it requires is a little knowledge of the SMTP protocol and your programming language of choice. To prove just how easy it is, I'm going to walk you through the basics using a simple python script.
Warnings and Disclaimers
While this process will get you up and running, you need to be aware of the following risks:
- Do this too much and you will get put on a naughty list (e.g. Spamhaus), especially if you are using a dynamic IP address from your ISP.
- B2C addresses: this does not work very well against the big boys who have their own procedures and spam rules (e.g.hotmail and yahoo).
- Incorrect results: some mail servers will give you incorrect results, for instance catch-all servers, which will accept all incoming email addresses, often forwarding incoming emails to a central mailbox. Yahoo addresses displays this catch-all behavior.
This script on its own is not enterprise grade email verification; you will not be able to process millions of addresses with it.