If Outlook is re-acting very slowly and/or reporting that it needs to close due to an internal error then this is likely to be due to a data corruption in the PST (for POP3 style accounts) data file.

Outlook 2013 is delivered with an application called "scanpst.exe" that can be used to repair corrupted PST files

• Ensure that Outlook is fully closed
• Navigate to C:\Program Files (x86)\Microsoft Office 2013\Office15\Root\Office\etc. and locaet an application called "scanpst"
• Execute this program, you may need to run as administrator
• Click on the "browse" button, navigate to the 'Outlook Files' folder in 'My Documents' and select the problem PST file.  Click on "start".  It is strongly recommended that you opt to make a backup of the file
• After the file has been scanned, this can take 10-30 minutes or so, then it will report whether aby errors were found and if so, prompt you to repair.  Click on the "Repair" button