All E-Mail that arrives goes automatically into your INBOX.
If you DOWNLOAD E-Mail using a desktop E-Mail reader such as OUTLOOK, etc. - the E-Mail will usually be moved from the INBOX and placed into the "ALL MAIL" folder.
So, if you are using an IPhone, and you need to view an E-Mail that has already been archived or downloaded and is thus no longer in the INBOX - you need to
CLICK ON the BUTTON in the UPPER LEFT hand side of the Mail Screen until you get to a screen called Mailboxes.
Once there, you should see a List of "Inboxes"
Below that, you should see a List of "Accounts"
Here, under the Accounts, is where you want to choose your E-Mail account.
Once inside, you will see a list of FOLDERS.
Find the "ALL MAIL" folder, and open it.
That is where you will see ALL of your E-Mail - even the E-Mail that has already been archived or downloaded!