Do you mean that you have no Email accounts in the Apple Mail App?
if you have not, and have never had, then the Omni Disk Sweeper result is bogus but, if you do have an Apple Mail email account, remember that any attachments that come with your mail are stored, all your incoming and outgoing are stored.
The Mail App by itself is only 30.9MB
Remember that with your Time Machine backup you can always replace files should you find that they are needed, although Omni Disk Sweeper claims to be able to prevent you from doing that sort of damage.
Another helpful tool is JDiskReport available at http://www.jgoodies.com/freeware/jdiskreport
Hi, I am trying to make space on my computer. Omni disk sweeper tells me that my mail app is taking 25 GB of space even though no account is synced to the app. What is deleting those files can do? Thank you!