5.1.3 Bad Address Sytax error message

I have started to get an error message " 5.1.3 Bad Address syntax in my email account. I am using Outlook 2011. It does not appear to be affecting sending or receiving email. I have no idea what it means or why it has appeared, or how to resolve the issue. My internet provider is Optimum and I am using Airport Express wireless device.
Any help would be appreciated. The error sound "bong" is driving us nuts

Thanks