You can let others view or edit your mail, or open other people's mail if they have given you access.

How do I give someone access to manage my mail?
You can let specified people open your mail, calendar, to do, or contact entries. You can also let people send, edit (including adding a follow-up flag), and delete messages, create and delete calendar entries, and create and delete to do items for you.

How do I create a shortcut to another person's Mail?
You can create shortcuts to someone's Notes® mail, calendar, to do, or contacts, if that person has given you access.