There are many excellent websites that offer tools for checking your server's IP against blacklists, verifying DNS settings, and checking the general health of your server. However, with a cPanel server, there is one issue that often gets flagged as a potential problem: the SMTP Transaction Time. Fortunately, this is a false-positive on cPanel systems that you can ignore.
When performing a scan of your mailserver, your server's SMTP transaction time will likely be marked "not good." These automated tools reach that result by performing a standard SMTP connection to the server and noting the time it takes for that connection to be established. However, on a cPanel server, we have the option "Introduce a delay into the SMTP transaction for unknown hosts and messages detected as spam" inside the WHM >> Exim Configuration Manager interface enabled by default, in an effort to fight spam being delivered to your server.
A user sending spam is likely using automated tools to send as many messages as possible in a short period, but a legitimate mailserver sending an actual email will have no issue waiting the few extra seconds to connect and send the message. The same problem exists with the automated online tools, as they often don't wait the full 15 seconds that is allowed for the transaction to take place.
This SMTP delay may cause other errors in your report, such as problems with TLS support or missing reverse DNS, as the test wasn't able to make a proper connection to the machine to properly verify these settings. All cPanel systems support TLS, and you can manually check the reverse DNS to ensure that is properly setup.
You can find additional details about the "Introduce a delay into the SMTP transaction for unknown hosts and messages detected as spam" option at our documentation page here: Exim Configuration Manager