It is a Mailbox. Start with M, ends with X and have never ending amount of letters.
Mailbox is a receptacle for receiving incoming mail at a private residence or business. It is also called letter box, letterbox, letter plate, letter hole, mail slot. It's types are:-
User mailboxes: User mailboxes are assigned to individual users in your Exchange organization.
Linked mailboxes: Linked mailboxes are mailboxes that are accessed by users in a separate, trusted forest.
Therefore, Mailbox is the correct answer.
To learn more about Mailbox, refer here :
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