Huge mailbox access, and multiple mailbox access in Exchange Online/Microsoft 365

We are having a problem in Exchange Online/365.  I'm not sure it can be remedied, but wanted to see if anyone has any advice.  

We recently migrated to 365.  We have an office here with 4 users, and besides their own mail, they also manage seven senior officials mailboxes.  We get frequent FOIA requests of these mailboxes.  

The head of this department, when we still had a premise Exchange server, had her local profile set up with her email, and all seven of the other mailboxes as well using the "open additional mailboxes" option.  Her mailbox is 20 Gb, and one of the other boxes is 5Gb.  She has to constantly move between the mailboxes, moving items between the accounts.  There are several hundred, may thousands of folders.  This worked ok on the premise server, but now with us being online, of course we are having trouble.  

She tried to have it setup in a similar manner, with all of the other mailboxes listed in the "open other mailbox" option.  However, when accessing the mail, or trying to move between folders, her pc locks up, gives her memory errors, and is basically unusable.  Her memory is maxed out.  I've contacted Microsoft, and they don't have a solution.  

Does any one have any suggestions?  I don't think we're going to be able to replicate the old environment.
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Vasil Michev (MVP)Connect With a Mentor Commented:
That's what the Archive is for, ask the to move older items to the archive or simply force some more restrictive retention policy on that mailbox.

If this is not acceptable solution, you can try Outlook in Online mode and disable caching of shared foders as well. It's not gonna be fast, I promise you that, but at least it will be workable. Alternatively, she can use OWA to access the primary mailbox or at least the shared ones.
Jamie McKillopIT ManagerCommented:

Is her Outlook in cached mode?

TOHITAuthor Commented:
Yes it is.  Some people suggested unchecking the option to download folders, but then she can't access the folders, which is what she wants to do.
Jeffrey Kane - TechSoEasyPrincipal ConsultantCommented:
Just because her mailbox is 20GB does not mean that the offline cache is nearly that big.  Outlook now only downloads the last 12-months of messages for the local offline cache.

The problem, most likely is that if there are several thousand folders, Outlook will not actually sync those folders until the first time they are accessed.

Another possibility is that she doesn't have enough storage space on the hard drive of her computer.  Make sure that there is at least 60 - 80 GB of FREE SPACE on the drive which her Outlook files are stored (usually in C:\Users\<username>\AppData\Local\Microsoft\Outlook == but you can designate any location if there is another drive with more room on it).

Having plenty of free space overhead allows the computer to more quickly move the files around as necessary.

The last possibility is that your AntiVirus software is scanning the Outlook .ost files.  This is not necessary and will definitely slow down the operation of Outlook.  So be sure that these files are excluded from any AV scanning as AV software cannot scan inside of this database file anyhow.

If none of the above help, I would suggest that when managing the other user's mailboxes she try using the Outlook Web App instead.  It will provide much of the same functionality as Outlook, but she'll be working with the messages directly on the server instead.
TOHITAuthor Commented:
After trying almost everything recommended by Microsoft, we ended up using a combination of archiving and common folders.  Her process had to be changed, which wasn't necessarily a bad thing.
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