Mail was unable to verify the authenticity of the S/MME certificate

A couple of weeks ago, I started receiving the following error (in Apple's Mail app on my MacBook Pro running MacOS 14.7.4) every time I attempt to send email from my iCloud account.

Mail was unable to verify the authenticity of the S/MME certificate provided by "xxxxx@icloud.com". Messages signed by this user may be coming from a different source.

Is this something nefarious going on? Do I need to generate a new certificate in Keychain Access? I started to do this and got a little overwhelmed by all the various options I need to choose from.

Also, I clicked the "Details" button and this may or may not be helpful in a determination:

"This certificate has not been verified by a third party"