Apple Script to click the "Load Remote Content" button. I like to disable all remote content in Mail.app, and then bind this script to a keyboard shortcut using Keyboard Maestro. https://www.keyboardmaestro.com

load_remote_content_mail_dot_app.scpt

tell application "System Events" to tell process "Mail"
	set mainWindow to a reference to the first window
	set rootSplitter to a reference to the first splitter group of the mainWindow
	set firstSplitter to a reference to the last splitter group of the rootSplitter
	set scrollArea to a reference to the last scroll area of the firstSplitter
	set scrollGroup to a reference to the first group of the scrollArea
	
	if number of groups of the scrollGroup is greater than 1 then
		set maybeRemoteContentGroup to a reference to the first group of the scrollGroup
		
		if number of buttons of the maybeRemoteContentGroup is greater than or equal to 1 then
			set maybeRemoteContentButton to a reference to the last button of the maybeRemoteContentGroup
			if name of the maybeRemoteContentButton contains "load remote content" then
				click the maybeRemoteContentButton
			else
				name of the maybeRemoteContentButton
			end if
		else
			UI elements of maybeRemoteContentGroup
		end if
	else
		UI elements of the scrollGroup
	end if
end tell

load_remote_content_mail_dot_app_seperate_window.scpt

tell application "System Events" to tell process "Mail"
	set mainWindow to the first window
	set mainScrollGroup to the first scroll area of the mainWindow
	set everyMessage to every group of the mainScrollGroup
	
	log (number of everyMessage)
	repeat with currentMessage in everyMessage
		set loadRemoteContentButton to the first button of the first group in the currentMessage
		click the loadRemoteContentButton
	end repeat
end tell

Thank you for this script! It was driving me crazy that there is no menu item for this action.