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?
Check that your are not keeping a copy of every email you've received, with their attachments. Under Mail / Preferences / Accounts, by selecting your account and then Advanced, you'll find an option regarding this matter.
There are several options which may help you saving some space.
I think the problem is because of the IMAP folder synching and the attachments contained in e-mails in this folders: Attachments in folders are not only pointers to the original file, but synchronized (thus downloaded and stored on your hard disc) for each of the IMAP/GMAIL-folders one E-Mail is in. So if one e-mail is in 5 folders, the attachment will be stored 5 times. I'm not 100% sure about that, but this is my guess.
So head to the Webinterface of Gmail, open the settings and find the "labels" tab. Uncheck all unnecessary folders, so that they will not be synchronized between Mail.app and Gmail. Now much less disc space should be used.