503 START TLS command used when not advertised Port: 25 Secure(SSL)

Email sending via email-client failing with error An unknown error has occurred. Account 'mail.domain-name.net'. Server: 'mail.domain-name.net'. Protocol: SMTP, Server Response: '503 START TLS command used when not advertised'. Port: 25 Secure(SSL): Ues, Server Error: 503, Error Number: 0x8000CCC65

This error is due to the email client (outlook, xmail, thunderbird, outlook express... etc) trying to use "SSL" SMTP connections to connect to mail-server using mail.domain-name.net. Currently the domain mail.my-domain-name.net DOES-NOT have SSL enabled on it.

To verify check,
https://mail.domain-name.net - ssl page will not load
http://mail.domain-name.net - non-ssl page loads fine

There are two options to resolve this;

1. You can user the server's hostname as POP , SMTP or IMAP server in your email client settings (mentioned in your 'Account Activation' email)

2. You could disable the SSL checkboxes / selections in the email client account-settings and connect to mail server without SSL encryption.