How Do I Test Bounces Without Sending An Email?

You certainly can test your mail deliverability yourself, although it is a manual process that may require some setup on your end.

In short, you will be using Telnet port-25 sessions to simulate SMTP exchanges between your Linode and a mail server.

Please note, Telnet is not encrypted and not as secure a protocol as SSH.

While it is generally safe to be using Telnet for the purpose of testing mail deliverability/manually testing connections over X port, it is not considered to be a safe or secure protocol to use for other purposes, such as logging into your Linode.

The Telnet package is installed by default on Linode’s Debian/Ubuntu distributions and can be located at /usr/bin/telnet. If you are using another distro such as Centos/RHEL, Fedora, etc., Telnet can be installed by entering the command sudo yum install telnet. You can always verify if Telnet is installed by running which telnet to show its location on your system.

Now that Telnet is installed/verified, you will want to attempt to connect to the mail server, using the command telnet <mail server address> 25. Once connected, use HELO {your domain name} (specifically HELO with one L) to verify your connection.

Finally, the commands mail from:<{your email address}> and rcpt to:<{the email you are verifying}> are used to simulate sending email to the mail server and receiving mail from that server, respectively. For more information and detail about the process, I’d recommend checking out this blog post about Verify an Email Address Without Sending an Email.

Something else to keep in mind is that new accounts typically have SMTP filters enabled on ports 25, 465, and 587 by default. Unless these restrcitions have already been removed by Linode, the test method listed above will not work. You can request the removal of these port restrictions through a Support ticket citing your reasoning and use case.

How does this website https://h-supertools.com/email/email-validation allow free registered visitors to perform the test for free?

When I loaded the h-supertools site, my screen was overwhelmed with pop-ups, and the site required me to sign-up before being able to use their tester. If I had to guess, their tool is free because of the ad revenue generated from site visits and signups. To quote the internet adage: “When something is free, you are the product”.

You can automate this somewhat with perl… However, you need to be prepared for some frustration because it's not ever going to be foolproof…your 'bot is certainly not going to be aware of any/all the security checks the recipient may have in place.

Also, the procedure is slightly different if the recipient is using smtps (port 465) and/or submission (port 587). There is a certificate key exchange (required by TLS) that needs to happen.

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