System keychain prompt when sending email.

I have a certificate associated with one of my Mail accounts. When I send an email using this account, I am repeatedly prompted to enter my username and password. The dialogue box says "Mail wants to make changes. Enter an administrator's name and password to allow this. Mail wants to use the System keychain." This prompt occurs periodically while I am composing the email and exactly three times in succession after selecting send.

I've dealt with this for years under many versions of MacOS, tried most of the the recommendations I could find online, and done several trouble calls with Apple support. There was a recent post with what seems like the same issue to which the accepted response was "remove the certificate or reinstall with the correct permissions." Removal is not an option for me and I'm not sure what would be needed to reinstall with the "correct" permissions.

Any help is appreciated. Thank you.