andrewNZ: Pretty much (as per my previous post).
Machines sort 99% of printed mail and about 75% of hand written mail (they'll read things people struggle to read, but I'm yet to write an address the machine can read...), and they process roughly 10 items a second.
Id suggest the chance of someone tampering with the result at post is equal to someone tampering at the electrol office who actually open them.
For the OP's peace of mind, we even find loose ones, we bag them up and send them too.
This kind of attitude is also used to justify spying on people, who have "nothing to hide".
I'm actually quite surprised that it's the prevailing attitude here on GZ.