What is being verified:
- Valid domain:
- Valid user: verify if the user and mailbox really exist
How does e-mail work?
When an e-mail is sent, the following actions are taken:
First the destination mail server is determined by asking the DNS-server which server is responsible for the domain. The domain is the part after the @-sign in the e-mail address.
Next, an SMTP connection is made to that mail server (or, if it's not available, to the next mail server that is responsible for the domain). This is where the e-mail gets transferred to the recipient's mailbox.
Finally, the recipient will find the mail when checking the mailbox (e.g. using a POP3 or IMAP-client).