How do I "shred" individual emails in Microsoft Outlook?

We need to first clarify what you mean by “shredding”, and then I’ll explain
why, in general, an external shredding tool can’t work with Microsoft
Outlook.

And then I’ll explain what you can do instead.

What most people refer to as “shredding” a file is nothing more than a
secure delete. The file is first overwritten multiple times with specific
patterns of data so as to thwart any advanced data recovery techniques, and
then it’s finally deleted. There are both commercial and free tools to do this; my
preference is the freeware sdelete
utility from Microsoft and SysInternals.

The problem here is simple, though: shredders or secure delete utilities
work on files.

Outlook does not store individual messages in individual files.

Instead, Outlook stores all of your email, calendar, contacts and the like
in one big file, called a “PST” for Personal Store.

That means that you can’t “shred” an individual message, because there’s no
separate file to shred. The message is just some data somewhere inside of your
PST along with other information.

So let’s look at the goal and devise an alternate way to achieve roughly
the same thing.

The goal is simple: to delete a message from your hard disk in such a way
that even advanced data recovery techniques cannot find it. That typically
means obliterating with a secure delete or disk wiping type of function the
hard disk sectors in which the deleted message was stored.

Since we don’t know exactly what sectors those might have been, we have to
take some extra steps.

Here’s what I’d do for maximum security:

• Delete the message in Outlook. As we know this doesn’t
  actually delete the message, but moves it to the recycle bin with your Outlook
  PST.

• Empty Outlook’s recycle bin. This also doesn’t really
  delete the mail, it simply marks the space previously occupied by the email as
  now being free and available for Outlook to put new messages and other
  things.

• Exit as many other programs as you can. We want to make
  sure that the disk is used as little as possible for the next few steps.

• Compact the PST. This really does delete the messages
  because it removes all the unused space from the PST. The unused space in the
  PST is returned to the operating system as actual free disk space.

• Exit Outlook. We’ll be making a copy of the PST shortly and
  we need Outlook to stop accessing it while we do so. We also need to make sure
  that it’s not changing the PST during the next step.

• Wipe the hard disk free space. Using a tool like sdelete,
  or some of the other free-space wiping or shredding tools, this ensures that
  the unused space on your hard disk is securely wiped clean. No files that used
  to be stored in the free space can be recovered. Part of the compaction process
  will have “moved the PST around” on disk, and the disk areas that used to hold
  your message could be part of the free space – hence you’ll want to wipe
  ’em.

• Make a copy of the PST. Locate your PST and in that
  same folder simply make a copy of the PST under a new name. The copy
  will make use of the free space we just wiped clean. Make sure you’ve done this
  properly before proceeding.

• Shred or secure-delete the original PST. The problem this
  solves is that during the compaction process the PST may have been rewritten on
  top of the disk space formerly used by the message you’re attempting to delete.
  That means that the secure wipe we just did of free space won’t have
  obliterated all traces of the message. That’s not good enough, and you actually
  want to shred or secure delete that region as well by shredding the PST.

• Rename the copy you made of the PST back to the original
  name.

• Restart Outlook

Unfortunately, based on other disk activity that still may have been
happening while you were doing all this, there’s still a tiny chance that a
disk sector that held the message may have survived. If the sectors previously
occupied by the message were taken by another program between the time the PST
was compacted and the free space wiped, then that sector would never have been
securely shredded. That’s why shutting down as many applications as you can is
important in this process.

The good news, though, is that the chances of that happening really are
tiny. This process will give you a fairly high degree of confidence that the
message you’re attempting to shred is almost certainly, irretrievably,
gone.