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The envelope information for an email, much like a real-life envelope, describes who it should be delivered to, and who it came from. This differs from the 'To' and 'From' addresses stored in the headers of the email.

When an email is delivered to a server, it will only ever look at the Envelope information. Continuing the real-life analogy, this is much like the way a postman would look at the address on the front of a letter. When the email reaches its final destination, the envelope information is discarded. The header information is equivalent to the address info that is commonly written on the top of a real-life letter. It may say something completely different to the envelope 'From' address. The envelope is the determining factor in delivering the email to the correct location.