Solved: RAC0225: Sending the test mail failed - iDRAC 9

 Recently when configuring the SMTP settings on iDRAC 9 controllers, I kept receiving this error when testing:

RAC0225: Sending the test mail failed

The most common solutions found online involved making sure a DNS iDRAC Name and Static DNS Domain Name were set in iDRAC Settings -> Connectivity -> Network -> Common Settings.
However, these did not work in my case. Fortunately, I found the issue reviewing the support article for Configuring Email Notifications for iDRAC
Near the bottom of the article is this sneaky bit of info:

Note: After iDRAC is upgraded to version 4.00.00.00, you may stop receiving encrypted email alerts from iDRAC, if the external email server does not support encryption. iDRAC firmware version 4.00.00.00 introduces a user-selectable encryption option and the default protocol is StartTLS. To start receiving email messages again, disable the email encryption by using the following RACADM command: racadm set idrac.RemoteHosts.ConnectionEncryption None

In my case, the servers were shipped with version 4.00.00.00. If you're not familiar, you can use SSH to connect to the iDRAC9 (enabled by default). Use an app like PuTTY to connect to each iDRAC, log in with the same credentials used for the web console, and run the command: racadm set idrac.RemoteHosts.ConnectionEncryption None

A reset is not needed, simply test again and you should by greeted with a success message in iDRAC as well as an email from DNSiDRACName@StaticDNSDomainName with a subject of

Test email alert from DNSiDRACName

Stay safe!