previously outlook not closed properly

I am using outlook 2003. pst size is around 600 mb. After we got the mesage “outlook was not closed properly” outlook will start in safe mode first” so we allowed to start it in safe mode. after that we closed outlook and started in normal manner. it worked. But this is coming daily. We ahve updated office patches, outlook patches, we have run scanpst to correct database integrity.But still this problem comes frequently.
Please help. This problem is faced by Director of company and I am in trouble.