MailTester does not send any email to check the validity of an address.
We are just using queries sent to SMTP servers to test the validity of an email address.
Those commands where defined in 1982 by he Internet Engineering Task Force under RFC 821. You can find them here.
Not always. Some mail servers will relay an email regardless of whether it's sent to a valid or invalid recipient. These are known as Catch-All domains. Fortunately, about 75% of all email addresses can be verified.
We notify you of email addresses that cannot be tested with this icon: 
On the 75% of all email addresses we can test, about 50% are unvalid. By removing them from your mailing list of from you recipients, you will save 4g of CO2 equivalent for each email you are trying to send.
No, we retain no information or email address submitted on our server.
No, none of your checked emails are stored at our server side.
We are not storing any single personnal data about your visite.
You can access MailTester.Ninja's unlimited features by purchasing an encrypted key without the need to log in or provide personal details.
Reduce the size of attachments, for example by compressing images or sending a hyperlink instead of a document
Unsubscribe from all unnecessary newsletters and subscriptions
Do not keep old e-mail addresses in which unread mails pile up
Reduce the list of recipients to what is strictly necessary
Regularly sort your mailbox and eliminate all unnecessary messages
Use an application that cleans your mailbox