With the latest upgrade (Ventura 13.2.1) whenever I try to send email using the Apple client, I get a warning that the message can't be encrypted and I need to override the feature.
I previously had a self-signed certificate installed (I have no need to pay for a verified root cert) that I have deleted, but this did not solve the problem.
Is there any way to disable Apple email encryption?
MacBook Air 13″, macOS 13.2
P. Phillips, thanks for your reply. This was more complex than implied by the response. I had previously installed a single self-signed certificate which appeared to be in "My Certificates" under Local items, which I had removed before posting.
It turns out that MacOS appears to install multiple certificates scattered over "Certificates" and "My Certificates" as well as in Default and System keychains. Though I only installed one instance of a self-signed certificate, I had to remove a number of them (I didn't keep count, just removed anything that was identified as self-signed by me).
I'm sure there's some reason for this replication of certificates, but it's not clear to me what that would be.
I also removed the public and private keys related to the CA ascribed to me. That seemed to take care of the warning message.
P. Phillips, thanks for your reply. This was more complex than implied by the response. I had previously installed a single self-signed certificate which appeared to be in "My Certificates" under Local items, which I had removed before posting.
It turns out that MacOS appears to install multiple certificates scattered over "Certificates" and "My Certificates" as well as in Default and System keychains. Though I only installed one instance of a self-signed certificate, I had to remove a number of them (I didn't keep count, just removed anything that was identified as self-signed by me).
I'm sure there's some reason for this replication of certificates, but it's not clear to me what that would be.
I also removed the public and private keys related to the CA ascribed to me. That seemed to take care of the warning message.
Problem is solved by deleting what appears to be a multitude of copies of the same certificate.
How do I remove a certificate from Apple Mail?
By the information below reverse this process to Remove the Certificate
If not done properly as in Remove the Certificate from Keychain >> try again
Use personal certificates in Mail on Mac
If you want to send and receive signed and encrypted messages in the Mail app 
on your Mac, you need a personal certificate.
Step 1: Obtain a certificate
For each email address you want to use to send signed messages and receive encrypted messages, you must get a certificate from a certificate authority (CA). See Request a certificate from a certificate authority.
Step 2: Import the certificate
To import the certificate into Keychain Access, double-click the certificate file you received from the CA. Once you import your certificate, it should be listed in the My Certificates category in Keychain Access.
The certificate file must have a file extension that indicates it contains certificates—such as .cer, .crt, .p12, or .p7c—or Keychain Access can’t import it.
If Keychain Access can’t import the certificate, try dragging the file onto the Keychain Access icon in the Finder. If that doesn’t work, contact the CA to ask if the certificate is expired or invalid.
Step 3: Use the certificate
Open your certificate in Keychain Access and make sure its trust setting is Use System Defaults or Always Trust. Now you can use the certificate to send and receive signed and encrypted messages.
Note: If for some reason your certificate isn’t associated with your email address, or you want to use the certificate with a different email address, Control-click the certificate in Keychain Access, choose New Identity Preference, and provide the requested information.
To use your certificate on another Mac, import the certificate into Keychain Access on that Mac.
Yup and refer to below image of 1 Certificate from Apple that appears several time.
That is without taking into account Third Party Certificates the User purposefully installed.
It was not implied the Third Party Certificate would be placed in only in One Location within Keychain
As I mentioned, I had already removed the certificate. This had no effect on the warning message.
So All is Good again ?
Disable email encryption by default
