You are connecting to the remote server with "smtp+tls" -- this is a mandatory TLS connection. The issue is either:
- The remote server's certificate is not accepted by your local server.
- The remote server requires a valid certificate from your local server.
As a next step, I would run smtpd(8) with -dv to try to obtain further clarity as to which is the case. If the results from that are unclear, then I recommend contacting your remote server's technical support team. For example, using "tls no-verify" might be a temporary solution but you would want to determine if there is a protocol or cipher issue between the two mail servers.