Create an email message in Mail.app with this AppleScript. Callable from the Terminal.

gistfile1.applescript

on replace(input, find, replacement)
  set text item delimiters to find
	set ti to text items of input
	set text item delimiters to replacement
	ti as text
end replace

--Variables
on run argv
	set recipientName to item 1 of argv
	set recipientAddress to item 2 of argv
	set theSubject to item 3 of argv
	set theContent to item 4 of argv
	set theContent to replace(theContent, "\\n", return)
	--Mail Tell Block
	tell application "Mail"
		--Create the message
		set theMessage to make new outgoing message with properties {subject:theSubject, content:theContent, visible:true}
		--Set a recipient
		tell theMessage
			make new to recipient with properties {name:recipientName, address:recipientAddress}
			--Send the Message
			--send
			save
			close
		end tell
	end tell
end run