Was we move to Exchange Online there are new limitations on the size of attachments that you can both send and receive. That limitation is 25 Megabyte. This limitation applies to messages you send and receive internally within the company and externally to the Internet.
With this in mind, as your mailbox is migrated to Exchange Online messages larger than 25 MB will NOT be migrated. If you need the attachments associated with these large messages you will need to save them. You can save them to your local drive, to your personal file share, or OneDrive.
This page describes how to search for and save attachments from Outlook and from your Enterprise Vault archive. Keep in mind that nothing is archived until it is 30 days old. Attachments younger than 30 days will be found in your Outlook client by using the Outlook Search. Items older than 30 days are archived and removed from Outlook and can be found by searching through your Enterprise Vault Archive.
Searching in Outlook
Outlook 2010 and Outlook 2013 are very similar when it comes to searching for items. When you open Outlook and move your mouse inside the search box then click, the menu changes at the top of the Outlook window.
From the new menu, select Search Tools and then select Advanced Find.
In the Advanced Find window select the More Choices tab and then select the Browse button next to Look In box.
In the Select Folder(s) window select the top level folder and unselect any other folders. Also select the Search subfolders box.
In the Advanced Find window change the size to greater than 25000 kilobytes (25 MB). Click on Find Now to start the search.
The search will display any messages in your mailbox greater then 25 MB. It will also show you what folder it is located in.
To save any attachments double-click on a message and when it opens right-click on the attachment you want to save and choose either Save As or Save All Attachments.
Searching Your Enterprise Vault Archive
To search you Enterprise Vault archive open your web browser and go to https://ev.ahss.org/EnterpriseVault/Searcho2k.asp. Select your name from the dropdown and click on Advanced Find…
In the Advanced Find window open up Browse…
Select the checkbox next to your name and also next to Search Subfolders… Select OK.
In the Size section choose to search for items bigger then 25 MB. Then click Find Now.
It may take several minutes for your search to complete. When it is done you will see the results and the number of matches found.
Open the item that you are interested in by clicking on the link.
After the message opens right-click on the attachment and select Save As or Save All Attachments.