If you add another Exchange account to Outlook then press the 'New Email' button whilst within the Inbox of that account then (correctly) the new email will be From that email account... BUT
If you instead add that account as an Additional Mailbox (mail settings, advanced) then when you create a new email whilst within the Inbox of that account then the From will be your Default Account.
This is driving me insane because my manager needs access to 5 email accounts, and occasionally Outlook asks him to enter the passwords again (usually when he travels abroad) but he never remembers the passwords and wanted me to write them down for him! I thought this was a bad idea so instead I added the email accounts as 'Additional Mailboxes' but now he has to remember to change the 'From' every time he writes a new email which is even worse!
Is it possible to make Outlook change the From address to the one he has selected when it is an 'Additional Mailbox' (not an additional Email Account), or maybe there is a way to stop Outlook asking him to login again at random and seemingly pointless intervals?