Frequently Asked Questions

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).

Is e-mail guaranteed to either be delivered or bounced?

No. There is no guarantee an e-mail is delivered, even if you didn't receive a bounce-mail.

There are several possible scenarios where you would not receive a bounce mail.

For example: when an SMTP server suddenly finds itself isolated from the Internet, it will not be able to send a bounce-mail.

Or maybe your address is not reachable so the bounce mail could not be delivered to you.

Is it possible that I can send e-mail, but not receive anything (or the other way around)?

Yes, sending and receiving are completely different things.

For sending you need to be able to contact the SMTP server.

For receiving you need to have access to your mailbox, usually via POP3 or IMAP. For mail to be able to arrive into your mailbox, your mailserver has to be registered in the DNS system, and it has to be reachable from the Internet.

Since sending and receiving are completely different actions, one can sometimes work while the other doesn't.

Where does come from?

A lot of people come to me asking if a certain e-mail address is having problems or why a certain e-mail address doesn't work. Usually the answer is in the bounce-mail, but often it is in such technical terms most people don't know what it means.

There are some routine tests I usually do to find out more about an e-mail address.

I decided to automate these tests, and make them available for everybody on this site.

How does work? looks up the SMTP server responsible for the e-mail address, and tries to establish an SMTP connection. It then tries to send several commands, almost as if an e-mail is going to be sent. When enough information is gathered, the connection is aborted.

Does always work?

No, not always. Some servers only connect to the internet at specified times to retrieve the e-mails that are queued at a secondary SMTP server. Even worse, some SMTP servers don't give any information about the validity of an e-mail address, so there's no way of knowing if an e-mail address is valid (except by sending an actual mail to it, and seeing if it bounces or not).

Is free?

Yes, is free for individual e-mail address when checked via the website.

Can check a whole database for me? has developed a program that can do this. Please click on Download for more information

Customer testimonials

Heath Atwood

Email Marketing Expert

When I signed up for, I was a bit skeptical at first. How much difference can it make for a relatively smaller company like us? Answer: a lot. We didn’t change our email strategy much still our email open rate has increased like 3-4%. I recommend

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Chief Marketing Officer

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We are a small startup and can’t afford to spend hundreds of dollars every month on email verification.’s flexible pricing plan works for us. And the service you would get is excellent.

Alex Lee

Sales And Customer Success Manager
AB Real Estate

Efficient, reliable and pocket friendly--- I love I have personally recommended this app to my entreprenuer friends.

Jordy Eubanks


Mailtester is awesome. We have big lists here with us and we keep them clean with It never disappoints.

Anaiah Whitten

Marketing Expert

I am a fan. is super accurate and easy to use. I set it up and forget about it. It’s doing its job without any issue.

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