Surreptious opening of most envelopes sent through the mail takes little effort and only minimal skill. Instructions are readily available on the Internet and in books such as "CIA Flaps and Seals Manual" (Paladin Press).

While your mail is in the hands of the postal service it is relatively secure from outside snooping but readily available to postal employees and the alphabet soup of government agencies that may, for whatever reason, take an interest in your private correspondence. If your mail is delivered to your home, perhaps to a box at the end of your driveway, someone can easily remove your mail before you do, unless you are waiting when the postment delivers it.

LET ME ASK YOU THIS...
If a private investigation or neighborhood snoop removed private letters from your mailbox, carefully opened them, recorded the contents, and then resealed them and returned them to your mailbox, would you know it?
If you received a letter in the mail on Wednesdays instead of tuesday, would you be aware that it had been missing that day?

HOW GOVERNMENT KEEP THEIR MAIL PRIVATE
When government agencies send sensitive material, they double-wrap it to prevent it's surreptitious opening. The government procedure is to place the sensitive material in an envelope, seal all seams and edges with tape, and record appropriate addresses and security markings on the envelope.

This envelope is then placed into a second envelope, which then has all edges and seams sealed with tape. The outer envelope is addressed, but there are no security markings recorded on it. This is effective, but the outer envelope with all edges and seams sealed with tape stands out from ordinary mail.

HOW YOU SHOULD KEEP YOUR MAIL PRIVATE
For security of personal correspondence I recommend double-wrapping
your private letters, but with a bit of a change. Instead of sealing just the edges of the inner envelope, i use clear sealing tape or clear packing tape.

1- Cut a lenght of tape twice the lenght of your inner envelope.
Place the tape on a table, adhesive side up. You may need two strips of sealing/packing tape to cover the entire width of the envelope (I was able to obtain 4-inch-wide packing tape, which works well for the smaller 3 5/8 x 6-inch personal correspondence envelopes, from a local moving company).

2- Now, after sealing your envelope, place it on the adhesive portion of the tape and then fold the tape around the envelope so that it covers all surfaces.Leave a slight edge of tape around the envelope so that you can have an adhesive-toadhesive seal. This type of seal, covering all surfaces of the envelope, is damn near impossible to get into in a surreptitious manner.

3- Now, place the sealed envelope in an out envelope and seal and address it normally.

The outer envelope doesn't stand out from other letters, but your private correspondence sealed inside is protected from snoops.

Even the old tricks of using chemical sprays to make the envelope momentarily transparent won't work because the inner envelope is completely wrapped in tape and therefore impenetrable by these sprays.