Why is my valid email being marked as invalid?

MailboxValidator Email Validation


In MailboxValidator, we have a field in our email validation results called is_verified which basically confirms that the email address is valid. Even though this value is True most of the time for valid emails, there are cases where is_verified is False but the validated email address is actually valid and active.

Before we go further, if you’re not familiar with MailboxValidator’s validation result fields, you may want to have a quick look at the page below to get up to speed.

Read about the validation result fields in MailboxValidator.


Reason for the is_verified returning False

As we have received many questions in the past from our users regarding the False value of the is_verified field, we thought it is good to go into more details about this anomalous behavior. Regular users of our email validation service should be aware that our validation is done in real-time via network communications with the mail server hosting the email address to be validated.

During said communications, our system will send a bunch of Simple Mail Transfer Protocol (SMTP) commands to the mail server to establish a connection. Then it will query the server to check if the email address in question does actually exist. The most important command is the RCPT TO command which indicates to the mail server which email address we are interested in. If the email address exists, the command will return a status of Ok.

The above occurs predictably in most mail servers. However, occasionally we do end up querying a mail server that is setup not to respond with an Ok status, no matter what. That is when we will mark the email address as invalid since we weren’t able to confirm the email exists.

Read more about SMTP commands.



The email validation process is not perfect. But don’t let that stop you from cleaning your mailing list. As a matter of fact, cleaning your email list regularly will help to keep your bounce rate to a minimum. This keeps your email sending cost down while helping to boost your open & click rates.



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