If you are like me you probably got very frustrated with the Win10 Mail app's inability to separate the username from the outgoing e-mail address (which are not necessarily the same and in my case they are in fact different). For a long time I have been complaining via the feedback app about this problem, but it seems MS has quietly fixed the issue on 10586.11 (or maybe even earlier)...

The trick is to delete your entire e-mail account setup for your IMAP account, and start setting one up from scratch using the 'advanced setup' then 'internet email' options. You will notice there is now an extra field to let you specify email address in addition to the username.

But the MS software engineers apparently rolled this out in a rush and forgot about updating the 'modify account' settings to let you see these extra email address field as well. Which means as it is now, once you have initially setup your email address field, you will never be able to modify it... hence the need to start setting a new account up from scratch.