Custom email addresses entered into Outlook New Email From: field are not saved as expected. The email is successfully sent using the custom From address, but the user has to enter it manually every time.
The issue seems to happen if there are multiple Microsoft Exchange accounts configured in Outlook's profile and the custom From address is used to send emails from a non-primary Exchange account.
Note: Primary Exchange account in Outlook is not the same as the default email account. Whereas you can change the default email account in Outlook account settings, the primary Exchange account is the first Exchange account that was added to the Outlook profile and cannot be changed.
To resolve the issue I had to create a new Outlook profile for the affected user making sure the relevant Exchange account was added as the first account.
Windows 10 Pro