This has been raised before but so far I have not seen a fix so I'd thought I'd post my observations in the hope that someone from Microsoft pick this up.
The Windows 10 Email app seems to have a problem with IMAP e-mail. Under Windows 8/8.1, when creating an account one had to enter an e-mail address and a username/password. Under Windows 10 a an account name - which can be a random text string - and a username/password are requested. The username can be an e-mail address or a domain username or something else depending on whether it used in a work envirnment, or whatever account username format is expected by the ISP.
A network packet capture on the sending computer during a failed send attempt shows that the Windows 10 Email app uses the username parameter to BOTH log in to the IMAP server and as the 'mail from' address in the SMTP transaction. If the username is not specified as an e-mail address, sending e-mail won't work, although receiving e-mail will work if the username/password is specified correctly.
The Windows 10 Email app does not appear to have a way of specifying the account e-mail address as was possible in Windows 8/8.1. Even if you specify an e-mail address in the Account Name field, this does not make a difference as this parameter does not appear to be used as the 'mail from' address.
Can I ask whether Microsoft have plans to fix this problem and attend to the other issues mentioned on this forum with this app and when that will be please? It would be very useful to get the e-mail app working properly. It would also be unfortunate if this problem were to be carried over on to the phone version of Windows 10.
Thank you.