Outlook error Ox800CCC69, Server Response '550 message = IP is listed at cbl.abuseat.org'

Mail client error:

An unknown error has occurred. Subject: FW:Favor, please,\" Account\"
info@domain.com , Server \"mail.domain.com\" Protocol: SMTP, Server Response
\'550 message = 65.121.95.66 is listed at cbl.abuseat.org\'  Port: 25, Secure
(SSL): No, Server Error: 550, Error Number Ox800CCC69

We are using Realtime Blackhole List (RBL) in our mail servers to keep away a good amount of spams coming
from servers that are already listed in abuse reporting sites like spamcop.net, list.dsbl.org, cbl.abuseat.org etc.
The above error message shows that your ISP's IP address is listed in one of those sites.

If you are a Deru Customer, you can bypass this RBL checking by enabling authenticated smtp in your mail client.

For enabling authenticated smtp, please refer the following article.