Unable to send emails error 550 5.7.1 Message rejected due to content restrictions

I'm having trouble sending to emailing, we've removed all signatures, logos, links and were sending in plain text but the emails keep getting bounced back with error: 550 5.7.1 Message rejected due to content restrictions. It's only happening when we send it to one organization (different people), worked before but no longer. Check with receiving organization IT and the didn't change anything to prevent them from receiving our emails. Using Outlook and our email host is machighway.