There used an Apple shortcut to select a specific from address in a Mail new message window.
My brother at PeoplePC/Earthlink usually doesn't receive emails from my domain. But he can from my iCloud account. I had a KM macro that when I typed 'ss' put in his email address and changed the from address to my iCloud account (using the Apple shortcut). (ss was convenient because it's short, related to his initials and not many words start with ss). This stopped working, I think before Sequoia, but I can't remember to always change the from to my iCloud address.
Summary: shortcut or KM macro to change Mail from account
Edit: nope, "shortcut" in the original version of the question referred to an Apple "shortcut". Thanks to OP for clarifying that by updating the question. Remember that when you say "shortcut" you are referring to a trigger for a macro. And you can't expect anyone to be able to help you debug a macro that only you can see the contents of. If changing the trigger doesn't help (as it probably won't), it's the macro itself that you need help with.
The following have tips on troubleshooting, and helping others to help you.
You can do this without a macro, using System Setting's "Keyboard Shortcuts". Click on "App Shortcuts", then click the "+" button. In the dialog that appears:
Set Application to Mail
Set the Menu title exactly as the "From" pop-up menu item shows it, remembering that the "-" is an em-dash (⌥-). For example My Name – m.name@example.com
Set the "Keyboard shortcut" to something that doesn't clash with others in Mail -- perhaps ⌃S in this case?
Click "Done", then "Done", then Quit System Settings
And you should be good -- make a new email in Mail, hit your shortcut key, and the correct account should be selected.
I'm not familiar with any built-in Apple shortcut, though there is the ability to create one in Settings > Keyboard > Shortcuts > App Shortcuts. Last I knew, the desired sending account had to be entered there in this format:
But it's possible the format has changed in newer versions of macOS. The requisite format to use in Settings can be seen in Mail in a new message window by looking at the pop-up menu choices, which would be seen where the sending address can be changed manually.
Once Settings has been used to create a shortcut for the preferred sending address, a Keyboard Maestro macro could be made for configuring the email as described.
A completely different way to do all of this is to have a Keyboard Maestro macro that uses the Execute An AppleScript action to create the message. The code for that just needs the receiving and sending addresses changed:
tell application "Mail"
activate
set thenewMessage to make new outgoing message with properties {visible:true}
set sender of thenewMessage to "Your Name <Name@Address.com>"
tell thenewMessage
make new to recipient at end of to recipients with properties {address:"Brother@Earthlink.com"}
end tell
end tell
If you are making "new" emails to send to your brother, using @NaOH's AppleScript in your macro is probably the best way to go.
But when you have an "outgoing" message already open, AS has get trouble referencing it -- that's where the Mail-specific Keyboard shortcut comes into its own.