It used to be a rule of thumb in direct mail that in order to get opened, envelopes had to first "get noticed". These days, particularly in business-to-business mail, that no longer applies. Why? Because the more your envelope looks like "junk mail", the less chance it has of making it through the mail room, executive secretary and other corporate "screens".

Can teaser copy ever work? Sure. But more often than not, because you identify the package as promotional in nature, you give the reader a reason not to open the envelope, rather than incent them to go further. Next time you're thinking of using envelope copy, test 25% of the names with a plain envelope (just your logo and return address).