user who needs to see all of the past emails of an email alias

I guess thats kind of a hard question because an email alias is assigned to each mailbox.  You can add them with full access to view the mailbox with the associated alias.  If this alias did not exist prior then the individuals who emailed the non existent alias on exchange would have recieved a bounce back stating it does not exist.  You can add multiple email alias to a mailbox so it recieves more mail.

Example

Mailbox - Test User
Alias - tuser@domain.com
2nd Alias - testu@domain.com

That mailbox will recieve email on both accounts so if i send an email to tuser and testu that same mailbox will recieve both messages.  If i send to testuser@domain.com it does not exist and it will get bounced back from exchange.

Does this user just need to see the email of another existing employee or an employee no longer with the company?

Another existing employee

I am using cashed exchange mode, perhaps I should just import the previous employee cashed exchange file.

I usually open up exchange, assign the user to the 2nd mailbox with Full Access.  Then add the user to Send As (if the user needs to send from that mailbox as the targeted email address).

You can open up exchange powershell and input the following:

Add-MailboxPermission "*Target User*" -User "*User*" -AccessRights FullAccess -Automapping $false

Open up outlook, click File > Account Settings > Email Tab > Change > More Settings > Advanced Tab

Under mailboxes click Add, add the target mailbox


This will add the mailbox to his account so the new employee will see the mailbox of the existing user.  They will also see new mail also.

Thank you