|One of two things I find frequently cause this ....|
1. You have too many messages stored in the inbox.
2. You have a corrupt .pst file that will need repaired.
Solutions are to move the messages from the inbox to another folder that you create in personal folders. The other resolution is to use the outlook tool to repair the .pst folder - location of the tool depends on the version of office. Go to C:\Program Files\Microsoft Office\ Office 14 and perform a search for the scanpst.exe file. Run and let it repair.
Lastly, check your account settings to verify that you have correct smtp and mail servers and that you have outgoing and incoming settings correctly chosen.