I assume that the error response is part of a non-delivery notification email you are getting from your outbound SMTP service.
If that's the case, it means that the recipient's server is configured to only allow email submission over TLS-encrypted connections, but your outbound server is trying to submit it over an unencrypted connection.
Even though you are able to submit email for delivery to your outbound server using TLS, this does not affect the subsequent communication between the two SMTP servers that occurs independently of this. That process is outside SMTP client's control.
To fix the problem, you would either have to configure your outbound SMTP server to use TLS when possible, or convince the recipient's server maintainers to relax their security requirements.