Mail means the official system used for sending and delivering letters, packages, etc.
In Oxford Advanced Learner’s Dictionaries, the definition of Official are
1.[only before noun] connected with the job of somebody who is in a position of authority
2.[usually before noun] agreed to, said, done, etc. by somebody who is in a position of authority
3.[only before noun] that is told to the public but may not be true
4.[only before noun] formal and attended by people in authority
Which one is suitable for this sentence?