If you are mailing something in an envelope, it is worth putting some thought into the outside of the envelope.
There is one crucial idea in mind:

Maximize the number of people who open the envelope.

If you can put something on the outside of the envelope that people will notice, which makes them curious, and which appeals to a vital problem or interest – then you will have a likely winner.
When designing an envelope, we start with this goal: Make opening the envelope irresistible to any serious prospect.