Question about Outlook Express

I have Yahoo Mail Plus imported into Office for Mac Outlook 2011. Everything has worked fine for 6 years until a few days ago. I received a [SYS/TEMP] problem retrieving message error popup when trying to Send/Receive new email messages. When clicking on that error message, it reads 'error code: -16999.' No email is able to be sent or received.

Any ideas on what I can do to get it working again? Thank you in advance!