If available, please provide any error details that may be seen by your e-mail account users when they attempt to send or receive e-mail.
It will help to know more detail so we can better diagnose what is happening:
1.) Are the e-mail account users able to receive messages without error?
2.) Are the problems seen only when users send?
3.) Are there any other specific problems other than outbound (sent) messages being detected as Spam by the recipient's e-mail service provider?
4.) Are both SPF and DomainKeys enabled via cPanel of the affected accounts?
As a starting point, I recommend ensuring that both SPF and DomainKeys are enabled via cPanel, at the following menu path:
cPanel: Mail >> Email Authentication
EmailAuthentication < AllDocumentation/CpanelDocs < TWiki