New Outlook 365 install only shows the Microsoft email account and not other pre-existing outlook 2016 accounts

The online OWA can take time to finish up, but that's weird Outlook configured it fine. Try other browsers too for this OWA test. 

Thank you for the fast response.  I will give it a bit more time, but nothing is populating so far after 1/2 hour.  I will try other browsers.

I've seen them take a few hours and some techs have reported 24 or more hours. 

Ok, I'll be patient. Combined they are fairly large files .  I added the 365 account to desktop app to get it to show.  Was that the correct procedure. Is there something I need to do on the 365 side.

I appreciate your help

A new mbx on MS365 admin center can take time for it to finish. Sometimes it's fast and sometimes hours. You should not be able to launch it from Outlook until you can see it in OWA.

Now you are mentioning files, what files? 

Hi Lisa,

Sorry for the confusion.  I was referring to the Outlook data file size which may not be relevant.

From the desktop app I can send and and receive emails, just can't see the user's other email accounts in OWA.

Did you setup the correct permissions? 

I'm not sure what permissions are needed.  The user has 3 email accounts plus the Microsoft 365 account.  No issues on the desktop, just in OWA.

You mentioned other people's mailboxes, I'm really confused on your problem here. Can you outline it a bit better? 

Yes and I apologize for not being clearer.  All the email accounts belong to one person and the first three were in her Outlook 2016 app.  She then signed up and added Outlook 365 via  Office 365.  I no longer see the Outlook 2016 program in her Win 10 computer.  We are trying to get all four accounts to show in OWA.

The desktop 365 app lists all 4 accounts.  When we access the online 365 account we only see the single 365 email account and none of the original 3 Outlook accounts.  

Please feel free to ask any questions to clarify.

You can't see 4 accounts in one OWA, unless all 4 are on the same domain and you share them. 
Outlook can view many email accounts from different ISP and platforms.

Does that help? 

Yes it does help and the accounts are not in the same domain.  Can multiple OWA be added in one office 365 account or is it limited to 1?  

OK, got it.  Thank you so much for your help.  Have a great night.

You too.