My Solution. I have been using this for a short while and it works fine. Win10 with Outlook 2016. I had a main Exchange account for most Mail and all Contacts and all Calendar items - with 4 other emails connected as POP accounts. I have 5 Exchange accounts now and each one has their own account - but it can be tweaked and it is just fine for me.
Setup method for my solution - connect all email accounts to Outlook (I have 2016) as Exchange accounts. Then go to outlook.com and login to each account using a browser. Go to 'settings', and then select 'forwarding and IMAP'. You will probably then have to login to the outlook.com account again (second check) and you will be asked for access to the stored passwords on your browser - provide outlook.com/MSoft with access for the browser passwords. When that is done, you then go back to the 'forwarding and IMAP' option in settings, and then click on 'permit forwarding'. Then forward that email account to your main email account by typing in that email address in. Do that individually for all your other separate email accounts that you had connected as POP - forward them all to your main account.
When done, all emails will come into your main exchange account email address (and the others) and you can reply and compose from there using that email address, or using whatever email address you wish to use. The default for a new message is the main account, so always check and click the 'arrow head' if needed to change from which email account you want to send from. Replies from all secondary email accounts will automatically go from the email being replied to (but always check). Sometimes there is a short delay between arriving at the old POP address and the main account - but usually at the same time.
Setting up the 'home page' in Outlook App on the laptop. Put the main outlook exchange account on the top of the page and open all the folders. Then drag and move all your private folders to below that main account. Then go to the first of the 'secondary' email account and click the 'arrow head' in the left side folders - this will minimises all the folders for that email account so that all you see is the email address (not all the folders). Then click and hold on that email account name and drag it down to the bottom. Do the same for all secondary email accounts in the Outlook App. That leaves me with a single page main Exchange account with all its folders (inbox, drafts, sent, etc) and then under that is my private folders for all stored emails, and then under that (which I have to scroll down to see), is all the other email accounts - I dont use to send/receive from thos secondary accounts. Open Calendar and then click to remove (not show) all the other calendars (empty for me anyway), and do the same with Contacts (also empty) - that gives a single page for both of them too.
Not perfect and clean, but very useable. It astounds me that the numpties at MSoft dont realise many people use more than one email address, but then again they are always 'up to something'. Perhaps they will offer a solution in Outlook365, as another 'lock in' to their subscription service which will cost you ($watever they want) in 2030. I will never allow all my emails to be 'owned and controlled' by MSoft by using an online platform as my mail App. If in the future they block my current solution, then I will move to Thundercloud.
This workaround is OK - and it does offer one big advantage. Using Outlook 2016 with one Exchange and multiple POP accounts, meant that any mail from a POP account that was identified by outlook.com as 'junk', was not sent to the Outlook2016 App. I had to go online to outlook.com every now and then to make sure they had not blocked a legitimate email. Now they are all there on the Outlook2016 App 'home page' - I just scroll down and take a look once a week or so - no need to check direct on outlook.com using the browser.