This response comes from the remote server.
The cause is typically that that server performs some spam-prevention checks on your server, and decides to reject mail from it.
The checks that it can do are typically of two types:
1) HELO name
when a mail server connects to another on to deliver mail, it 'greets' the other server with its own name. This should correspond to the internet name of the server. Ex. mail.mydomain.com . So a DNS lookup to mail.mydomain.com should return the IP address of your server.
Some mail servers perform this check, and if the HELO name does not correspond to your IP address, they reject your mail.
DeskNow uses the name of the computer, as set in the OS, for the HELO greeting. But if you have not configured correctly the hostname in your computer, the HELO name will be wrong.
So you can either set the correct hostname for your server (how you do this depends on the OS you're using), or set the name directly in DeskNow. To do so, simply login as admin and set the server name in the Administration/Mail/General page.
The name must be a fully qualified domain name, like server1.mydomain.com . server1 (or whatever is the name in your case) must be a valid record in the DNS records of mydomain.com , and it must point to your mail servers' public IP address. Note that if your users connect to DeskNow from the Internet at the address www.mydomain.com, you may simply use www.mydomain.com as the server name.
After this change, restart DeskNow. This will force DeskNow to use this 'fake' name in SMTP and POP3 connections.
An alternative solution is to use SMTP forwarding through your ISP's SMTP server. This can be done in Administration/Mail/SMTP Outgoing.
2) Reverse DNS record
Some mail servers (most famously AOL) also check that your mail server, as stated in the MX record of your domain, has a "reverse DNS" record, and that it points to your server. The reverse DNS is basically the inverse operation of a DNS lookup: given an IP address, it gives you the internet name.
You cannot setup your reverse DNS record yourself. You need to ask this to your ISP. Simply asking "I need to have the reverse DNS record for my IP 188.8.131.52 to point to the name server1.mydomain.com" should do. (Use the appropriate IP and name). If your ISP cannot give you this service, you have two options:
1) change ISP
2) use SMTP forwarding to send mail through your ISP
Note that DeskNow can perform this check as well, against other servers. See the following article: /kb/idx/4/025/article/