There are two tasks for this:
1. Assign Full access permission to user B on User A’s mailbox
2. Give Send As/Receive As permission to User B on User A’s mailbox.
Full Access Permission:
Run the cmdlet: Add-MailboxPermission “UserB” -User “UserA” -AccessRights FullAccess -InheritanceType all
Receive As/ Send As permission:
Add-ADPermission “UserB” -User “DomainUserA” -Extendedrights “Send As”
Add-ADPermission “UserB” -User UserA -Extendedrights “Receive-As”
