I see this when I send email to an email account with new cpanel server.
The response from the remote server was: 550 Please turn on SMTP Authentication in your mail client. mail-***.google.com [####]:##### is not permitted to relay through this server without authentication.
- Check DNS records – make sure MX entries are pointing to CORRECT server
- Check ssl is on for account and certs are ready. Make sure at least 1 subdomain is accessible (or main domain) to get cert.
- If using autossl, will have to wait to get cert
- Turn on this: enabling WHM > Service Configuration > Exim Configuration Manager: “Allow weak SSL/TLS ciphers”
- If you are on Gsuite there is a cache so even if you fixed, the bounce back will continue. You can try another Gmail account instead.
- Best is to try an old school / private SMTP server that does not cache