I have 1 user affected by this issue. We replicated the scenario on several computer, different location with the same Outlook program version (MS Office 2010 STD) and we get the same result (See error below). But when we used OWA it let me send the email without problem. In addition, the recipient ensured that our domain is white-listed.
#< #5.0.0 X-Spam-&-Virus-Firewall; host smtp.secureserver.net[x.x.x.x] said: 552 5.2.0 EFRD1n00m4yi1yR01FRHir IB212 msg rejected as spam (in reply to end of DATA command)> #SMTP#