Thank you for your response. I tried the ExcludeLastKnownGoodURL key but not the others to be honest because I stumbled on the Microsoft Mail 'fix'. But let us get real here. Registry hacks to fix an issue shouldn't be necessary. Under normal circumstances, a non IT person would struggle with such changes and what we are doing is fixing Microsoft bugs with registry hacks.
I have never had an issue adding multiple accounts on Outlook for Mac where accounts having existed as non O365 accounts previously on the same email address, it just works, so clearly the way Microsoft Windows handles these accounts is bug ridden.
It should absolutely not be necessary to tweak the registry to fix a bug end of. What we are doing is finding ways around an issue that Microsoft needs to fix. Microsoft Mail was quite happy to add the account without issue so why not Microsoft Office Outlook 2016. Adding the account to Microsoft Mail allowed me to easily add the account to Outlook afterwards, a much quicker solution than registry tweak trial and errors. As IT admins we are constantly finding ways to fix Microsoft bugs and this really should not be necessary to make things work. As IT admins we choose this route because we know that Microsoft will not fix the issues we find any time soon. Even support requests to Microsoft directly have failed to fix this issue for some users.
Sorry rant over, but when you have multiple users having the same issue on multiple PC's the whole installation process for Office365 becomes very stressful especially when you quote the customer 4 hours to do a simple switch to O365 and it ends up taking 8 because you then have to research the issue, find the correct solution from a plethora of potential solutions and then apply the fix in the hope it works. If Microsoft were billed for every wasted IT admin hour to find workarounds they would soon be fixing these issues.