That error really just means:
"There is something in the message header that makes it look like spam"
The information that gets passed in the header is:
From Address
To Address
CC Address
BCC Address
Subject
Reply To
Return Path
Organization
X-Mailer
The most common cause of the problem is using a from address that is not a valid address for your host domain.
If you are using a dynamic binding fro the from address, change that so the dynamic binding is the Reply To, and use an email address for your domain as the from address. If you use both an from Name and a from address, this can cause an issue on some servers, use only the from address.
Double check the mail subject also, to make sure it does not include common email spam words like "password". the return path, organization and x-mailer are all optional, but you may want to enter your business name for the organization, this will help with spam rating. you can also set the X-Mailer to your servers php version, "PHP 5.2.3". adding these settings can help with spam ratings, even though they are optional.
For the return path, you can enter an email address to receive bounce backs.
