The BAK files your OE folders before compacting. They are copied there
as a precaution in case something goes wrong with the compacting.
1) Look in OE at Tools, Options, Maintenance, Store Folder for the
location of your Store Folder. Then close OE.
2) Go to Recycle Bin and restore the most recent Sent Items.bak file.
That will put it in OE Store folder, but still as a BAK file.
3) Open the Windows File Explorer and look at the store folder you found
in step 1. (You may need to enable the the display Hidden and System
files in Tools, Folder Options, View as well as uncheck the option to
Hide extensions of known file types. You should see both a Sent
Items.dbx file as well as the Sent Items.bak. Rename the Sent Items.DBX
to Sentold.dbx and then rename Sent Items.Bak to Sent Items.DBX. When
you open OE, your Sent Items messages should be restored.
Tarbomb: Someone asks you to extract a tar archive into an existing directory. This tar archive can be crafted to explode into a million files, or inject files into the system by guessing filenames. You should make the habit of decompressing tars inside a cleanly made directory
Decompression bomb: Someone asks you to extract an archive which appears to be a small download. In reality it's highly compressed data and will inflate to hundreds of GB's, filling your hard drive. You should not touch data from an untrusted source