If you are the manager of your entity, you can delete a mailbox from Administration settings > EACAT Procedures > Mailboxes > select a specific mailbox > icon ... > Delete mailbox
Deleting a mailbox has important implications, as it means disconnecting the associated services and users. Also, once deleted, the previous information cannot be recovered.
It is important to know that the global mailbox cannot be deleted.
Find more information at:
EGT - Can the global mailbox be deleted?
EGT - What are mailboxes?