Push the Envelope

Push the envelope.- To attempt to extend the current limits of performance. To innovate, or go beyond commonly accepted boundaries. (The Phrase Finder) / To push it to the limit. to see how far you can go with something without getting caught (Urban Dictionary).

It seems that envelope, at lest initially, had nothing to do with the mail word: read here about the origin. 

(By the way... Do you know Jean Wilson's blog Pushing the Envelope?)