The bug again though - happily fixed in Outlook on 'Redstone', I believe. All of this applies to production Lumia 650/950/950 XL and, notably, the mass of older Windows Phones from the S4-era (with model names ending, typically, in '20') which have been upgraded against Microsoft's advice to Windows 10 Mobile via the Insiders Programme and the 'Release Preview' ring.
So you add a Google/Gmail account to Outlook, intending to sync your Google contacts and calendar - I'm sure most people here have done this at one time or another. And the sync goes fine. Your Gmail mailbox in Outlook populates fine and there are all your Gmails waiting to be read. Wonderful.
Except that you then read your first email and try to reply. Typing goes well until you hit the wrong letter and you tap on the backspace 'key'. Nothing happens. You try again. Nothing. Puzzled you realise that typing still works in other applications and that it's Outlook that's at fault. So you restart the app. Still no dice. You restart the phone. Same. You're, in a word, screwed, unable to reply to emails!
At this point, the plot thickens. You're still able to reply to emails in your Outlook/live/Microsoft mailbox. WHY ON EARTH would the source of the Exchange or IMAP4 emails matter ONE IOTA to how the Windows 10 Mobile keyboard works? And yet, somehow, it does.
Microsoft hasn't seen this bug much because most of its internal testers have been using Microsoft accounts, naturally, yet type 'backspace' into the Windows 10 Mobile 'Windows Feedback Hub' app and you'll see dozens, if not hundreds of others have the same issue over the last year, off and on.
What's needed, of course, is for Microsoft to check the fixed version of Outlook Mail and Calendar works with Threshold (i.e. Windows 10 Mobile 10586.xxx) and then roll it out.
However, there's a simple (but slightly patience-straining) workaround that fixes the issue completely on affected devices: remove your Google account from the phone and add it back on again. Here's the walkthrough - there's nothing really to fear, just follow the steps:
Oh dear - not having backspace is a complete showstopper! (right) On with the workaround - from the hamburger 'Accounts' menu, long press and go into your Google 'Account settings'
Now delete your Google account from the phone - or at least from Outlook, you won't lose access to other apps which have used Google's OAuth for login.
Down to just my Microsoft account now - tap on the 'Accounts' header to get back into 'Manage Accounts and then tap on 'Add account'.
Pick Google, sign in and then wait. And wait.... while it all syncs down again from the Cloud.
In my case I gave up waiting after half an hour and figured that, actually, something had gone wrong. I restarted the phone and bingo, everything was there. Even better (right), I could now reply to emails and backspace with ease!
Such a bizarre bug. And I'm not too sure which Microsoft team to blame! Hopefully all this has helped someone else anyway. Roll on Outlook fixes and roll on Redstone, but only when it's all bug-free!