Outlook intermittent error

Few users are getting the below error intermittently almost everyday. They have to restart outlook to keep going.

Error:

When closing Windows search the outlook data file “…….\abc@abc.com.ost was not closed properly. Microsoft outlook must be restarted. If you continue to receive this error message, contact windows search support.

all the basic troubleshooting has been done, like indexing search, new profile for outlook etc. still no luck.