Problems with saving Outlook items

I have been using Outlook application for long days. Its PST file can store different types of files with this application. I got the following unknown error message last night when I go to save some of the Outlook items:
“Task 'Microsoft Exchange Server – Receiving' reported error (0x8004060C): ‘Unknown Error 0x8004060C'”
Many other associated errors Outlook starts prompting after this error. Outlook is running very slowly. I need help to fix this problem.
