Sent emails not syncing between two computers

I am syncing two different sub-email addresses, like aaa@address.com and bbb@address.com. One of the email addresses sync everything, but the other address, which I use most frequently, does not sync 'sent' emails, though it syncs everything else. I have compared the settings and accounts for both emails. They are set up the same. Mail is checked in iCloud. The two computers are powerbook with Mojave and airbook with catalina. How can I sync sent emails for this email address?