Error sending emails: ''

Hello,

I use Office 365 for Small Business, including email service from Microsoft, use Outlook for Mac as the client. When I send email to some specific external users, I get the following error message:

Remote Server returned '<xxx.yyy.com #5.4.6 smtp;554 5.4.6 Hop count exceeded - possible mail loop>'

I have tried to follow previous threads, but can not really find a solution for me specifically, not sure where I would go to do the FOPE changes or powershell scripts. 

Any help?

Thanks