Email Alias not listed in iCloud Mail

I just discovered that I have an email alias set up.

My email address is johnny.appleseed@me.com. I was surprised to discover that I can also send email to jappleseed@me.com.

I would like to use this alias. It’s shorter and simpler.

My question is:

When I researched email aliases I found references to managing it in Mail at iCloud.com or in my Mac’s mail preferences.

There is no trace of jappleseed@me.com in my email aliases on iCloud. (I didn’t even know this was a feature until yesterday). I have no aliases at all in iCloud.

It is possible that I set this up on MacOS a long long time ago. If so, it was probably pre-iCloud and could have been done on one of several Macs.

Before I start using this email address I would like to make sure that I can manage it properly.

Does anyone have any idea where I might look to find this? I checked appleid.apple.com. I thought it might be buried in my account somewhere. No luck.

Any help you can provide is greatly appreciated.