How to Access Archived Mailboxes
The following is based on the assumption your mailbox is so big we had to enable archiving for it.
In Outlook On the Web you will see your profile picture / initials in the upper right hand corner.
If you click this you can click Open another mailbox. A text box should pop up.
Enter the email address you would like to access. Assuming you have permissions, you will be brought to a new page where you will only see the shared mailbox's mail.
Once here, You can scroll all the way down on the left-hand side and you will see the In-Place Archive. That is presently where all archived items will go.
Archives are for historic retention for not commonly accessed mail items. It is done this way for performance on Microsoft's servers as well as our outlook clients.
Archive spacing can be increased to unlimited if necessary. Mailboxes cannot, however.