As “envelope” is a noun referring to a flat paper container for a letter or document, it's a bit challenging to find true antonyms in the traditional sense. However, we can think of some contrasting concepts.
Open space: While not a true antonym, it contrasts with the enclosed nature of an envelope. An envelope encloses and contains, while an open space is unrestricted and not confined in the way an envelope is. For example, “The letter was taken out of the envelope and placed in the open space on the desk.”
Unsealed area: This is more of a contrast than an antonym. An envelope is typically sealed to protect its contents, so an unsealed area is the opposite in terms of being open and not having the protection and containment that an envelope provides. For instance, “The papers were scattered on the unsealed area of the table, rather than being safely inside an envelope.”
Disorganized pile: Unlike an envelope which organizes and holds items neatly, a disorganized pile is messy and not contained. For example, “Instead of being in an envelope, the receipts were in a disorganized pile on the counter.”
Although these aren't strict antonyms, they offer a way to think about contrasting concepts to “envelope”.