Do you ever send them out in the Post?
Compared to (un-encrypted) email that anyone along the way can read (the email, or un-encrypted attachments); it's rather more secure.
You do have to pay though, but Docmail's "from 25p+VAT" pricing won't break the bank, and could improve response speed due to the impact of a physical version.
So it's worth considering us, or your Hybrid Mail provider of choice - though we're of course better, and cheaper than the rest
Cheers.
Will
p.s.
We do have a secure electronic documents system on the horizon, which is perfect for this kind of thing, but not ready yet

.