I found my startup disk free space to be very low and decreasing every minute without any files be downloaded or copied to my Macbook pro. I stopped running programs one by one and found when I stop Mail application (the original apple Mail) my free space is not decreasing anymore.
I had two email accounts assigned to my Mail application to be synchronized: my Gmail and the academic email account. I just removed Gmail from the Mail application and at once 40GB was restored to my free space!!!
And my question: While GMail has only 10GB quota (and I have used much less than this quota) how the Gmail account on the Mail application can capture 40GB of disk space? Is there any solution to use Gmail account active on Mail application, but with small disk space being used?