Apparently tries to resolve MX, connect and try to poke around. Surprisingly, the last time that trick worked reliably was in the early 2000s, because guess who also used the same trick? Yup, spammers - so most servers will either a) deny existence for any address, or b) claim existence for every address. And that's before greylisting and SPF and whatnot.
The only way to be sure that the address actually exists is to send mail and have the recipient actively respond - however, if there's no response, it can mean any number of things: no such address, message got lost, message got spambinned, message got deleted by recipient, recipient couldn't be bothered to reply.
In other words, do not expect certainty from e-mail.
The only way to be sure that the address actually exists is to send mail and have the recipient actively respond - however, if there's no response, it can mean any number of things: no such address, message got lost, message got spambinned, message got deleted by recipient, recipient couldn't be bothered to reply.
In other words, do not expect certainty from e-mail.