Do you have any logs you can provide to show the actual error you’re seeing and where it’s happening?
It sounds like you’re talking about Discourse sending emails via your DirectAdmin mail server. If this is the case, the sending domain has no bearing on SSL, the DirectAdmin server would just need a valid certificate for the domain used for DISCOURSE_SMTP_ADDRESS in your Discourse app.yml file.
When you added DISCOURSE_SMTP_OPENSSL_VERIFY_MODE to app.yml, did you also rebuild the container? Any change to app.yml will require the container to be rebuilt before it takes effect. Note however that even if that option does work, disabling OpenSSL verification is a security risk and should not be used unless the mail server is on the same machine or at least within the same closed network.