From: DeskNow Knowledge Base
http://www.desknow.com/kb/

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http://www.desknow.com/kb/idx/11/186/article/

If you're seeing this message in a mail delivery error, it typically indicates that DeskNow tried to deliver your message to a remote mail server, but this abrubptly closed the SMTP connection, because it considered you a spammer.

The most common cause of this problem is that the IP address where your DeskNow server is located is blacklisted by the receiving server (or by an antispam blacklist that they use).

Another common cause is if you use DeskNow to relay emails sent by a mass mailing program. Sometimes remote servers, especially those of large ISPs and webmail providers, block these mass mailings.

Solution

If your IP address is blacklisted, you can use SMTP Forwarding (Administration/Mail/SMTP Outgoing/Routing) to send emails via your ISP. IP addresses of many residential ISPs, and dynamic IP addresses, are often blacklisted, because easily abused by spammers.

An alternative is to change IP address, and/or ISP.

Note

If this problem occurs to all emails that you send out, regardless of the domain, it may be due to an interference by a resident virus scanner that tries to monitor SMTP traffic. It is recommended to diable email scanning functionalities in your antivirus, as they often interfer with mail servers, and are redundant since DeskNow can already invoke your antivirus to scan all your emails.