gistfile1.txt

tell application "Mail"
	set nextSelectedMessageID to missing value
	
	set selectedMessages to the selection
	
	if (count of selectedMessages) = 1 then
		set selectedMessageID to id of item 1 of selectedMessages
		
		-- Find the selected message's position in the list.
		set selectedMessageIndex to missing value
		set msgList to every message of message viewer 1
		set msgCount to count of msgList
		repeat with msgIndex from 1 to msgCount
			if id of item msgIndex of msgList = selectedMessageID then
				set selectedMessageIndex to msgIndex
				exit repeat
			end if
		end repeat
		
		if selectedMessageIndex = 1 then
			-- On the "last" msg: select the one previous
			set nextSelectedMessageID to id of item 2 of msgList
		else
			set nextSelectedMessageID to id of item (selectedMessageIndex - 1) of msgList
		end if
	else
		-- TODO handle multiple selection. Somewhat thorny.
	end if
	
	repeat with selectedMessage in selectedMessages
		set the mailbox of selectedMessage to mailbox "Read"
	end repeat
	
	if nextSelectedMessageID is not missing value then
		set x to item 1 of (every message of message viewer 1 whose id is nextSelectedMessageID)
		set selected messages of message viewer 1 to {x}
	end if
end tell