Replies to iCloud emails going from wrong address: Apple mail OS X

In my Apple Mail app I have 3 accounts: iCloud, GMail and Outlook. Recently my iCloud emails have become properly messed up because when I reply to an email to my iCloud address, it is often being sent from my GMail address. Hence when someone replies to that it ends up in my GMail inbox and the email trail is messed up.

NEW emails are sent from iCloud as expected - this problem is only affecting replies and appears to be random.

Looking in Prefs > Composing I have 'Automatically select best account' selected. However, the drop down only shows Outlook and GMail as alternatives - not iCloud.

I trashed the com.apple.mail.plist file and things improved. However, it is starting over again.

Aside from a new mail client - any suggestions?