Envelop is a verb. Envelope is a noun. Both words come from the Old French envoluper, but they came to English separately and have never been interchangeable. Even so, due to their obvious similarity ,the two words are often used in place of each other.
The shimmering zinc scales that envelop it echo the greasy grey skies above and the silver stretch of now quiet water. [Financial Times]
Her envelope had hearts where the o‘s in my name should have been and I tore it open and read her letter right there in the sun. [excerpted in NPR]
After the tour, we decided to start our one-mile climb up the mountain despite the heavy mist enveloping everything. [Washington Post]
Yes, look twice, and your iPad case could do double-duty as a chic envelope-style bag. [Vancouver Sun (article now offline)]
When the downpour ends, thick and eerily swirling low cloud envelops me. [The Australian]
Mitchell believes balcony gardening is all about being realistic about what you can achieve – but then pushing at the seed envelope. [Telegraph]