This isn’t a certificate problem, this is the other end telling you “I won’t accept credentials over a plaintext connection, you need to use STARTTLS to connect to me and authenticate so you can send mail.”
You should enable STARTTLS and either set up Let’s Encrypt for SMTP, or just disable certificate verification.
That’s because now you’re sending to the SMTP port which is used for mail delivery not submission. Unless that mailserver is configured to relay mail for your IP, it will only accept mail for the email domains for which it is configured to accept mail.