A popular e-mail client like MozillaThunderbird stores all the mails in dedicated areas if they are deleted. Even mails that have been deleted from the Recycle Bin or Trash, will have left a trail behind. For safety & security reasons, it is best that the mails be permanently deleted. This can be done through a manual intervention: one has to go to Compress Files under File option. This needs to be performed for each account you have. The best way is to configure Thunderbird software automatically to compress the files which will ultimately save some space in the disk Hard Disk.