I managed (under El Capitano) to set the new mail account with the following AppleScript (thanks to this post)
set outgoingEmail to "Sjoerd Op 't Land <sjoerd.optland@thedomain.com>"
do shell script "defaults write com.apple.mail-shared NewMessageFromAddress \"\\\"" & outgoingEmail & "\"\\\""
However, I feel it would be logical to have it available under Mail Actions > New Mail From Address. Unfortunately, I do not have the time and the skills to implement this correctly.
I managed (under El Capitano) to set the new mail account with the following AppleScript (thanks to this post)
However, I feel it would be logical to have it available under Mail Actions > New Mail From Address. Unfortunately, I do not have the time and the skills to implement this correctly.