SMTP (Simple Mail Transfer Protocol) is used by a lot of hosts and email systems all over the Internet. It facilitates sending messages between servers and it’s in the basis of POP and IMAP clients as it allows them to actually send emails. Sadly, as other technologies, while it has many good uses, it’s also often abused.
SMTP can be used for sending out spam or other unwanted mail. The good thing is that hosting clients can add SMTP restrictions to their accounts. This prevents other users from bypassing the mail server or using other methods to send emails. As a result, spam abuse is limited. And it provides the administrators with better control.
How to enable SMTP Restrictions with WHM
WHM is among the most popular hosting control panels out there. Many hosting providers use it and it makes the back end of the cPanel accessible. It’s not available to all users, but it does come with some very useful features. Of course, it supports SMTP restrictions.
Enabling SMTP Restrictions requires root access to WHM. So, it’s a feature for top level admins.
Login to WHM and look for the SMTP Restrictions link on the left-side menu panel. If you can’t see it, you can type SMTP into the search panel on top and click the SMTP Restrictions link in the results.
Click Enable. That’s it. Well, almost.
By default, the SMTP restrictions will allow the MTA, mailman and root to connect to remote SMTP servers. But, if you click on the Tweak Settings link, you will get a few other options.
Among them are even further restrictions or the opposite – allow to relay mail for IP addresses used by validated logins. Changing these options requires experience and they could cause some headaches for users. So keep in mind that you may have to revert or change them a few times until you achieve the desired results.
Enable SMTP Restrictions for Microsoft Exchange
This feature is also available for Microsoft 365, Office 365 and similar services. It has a different name, though: SMTP AUTH. “Virtually all modern email clients that connect to Exchange Online mailboxes in Office 365 or Microsoft 365 (for example, Outlook, Outlook on the web, iOS Mail, Outlook for iOS and Android, etc.) don’t use SMTP AUTH to send email messages”, says Microsoft recommending that admins disable or enable it only for specific accounts.
You can disable it globally with the following command in PowerShell:
Set-TransportConfig -SmtpClientAuthenticationDisabled $true
For more specific details you can check out Microsoft’s documentation. It’s definitely worth setting a few minutes aside to check the configuration of your setup and make sure you’re using the right settings.