Does Verify Addresses reliably report VALID USER vs. INVALID USER?
Yes, but there’s a third possibility. Once a valid domain is established (see the previous question), that domain’s SMTP server is used to detect whether that username is a valid user at the domain. This protocol results in three possible answers: either the username is confirmed to be a VALID USER, the username is confirmed to be an INVALID USER, or the SMTP server says that it will not reveal whether the username is or is not valid (certain companies have decided that reporting this information is a security violation). The later case results in a VALID DOMAIN report. The three answers are each 100% accurate, so VALID USER and INVALID USER reports are certain (there are no “false positives” or “false negatives”), and you will reliably get a VALID DOMAIN report when the SMTP server won’t tell. But because of the “won’t tell” answers, you can never detect all the valid users or all the invalid users. In our experience, you can find about 80% of the invalid addresses in a typical list.
