Mail scrolls are never generated randomly. You could acquire as many of these scrolls as you want by sending many messages to yourself, but they have little use in gameplay:
- They will not polymorph into other objects or be consumed by polypiling, they just turn into junk mail. 
- They cannot get wet.
- They are not affected by cancellation
- If you wish for or write a scroll of mail, or you discover a bones file with someone else's mail, it will be "junk mail addressed to the finder of the Eye of Larn". (The joke is that the roguelike Larn would send the user mail when they won.)
- A scroll of mail will "activate" an Elbereth square like any other object.
- They weigh as much as any other scroll (5), so you can become burdened by mail.
- Huge quantities of mail scrolls can be used for a shop-robbing exploit ("200 rock steal").