Get the account directory from Mail

I’m having a heck of a time getting the account directory that a specific message resides in.

The dictionary for Mail shows this property “account directory” but I don’t know how to access it.

account‚n [inh. item] : A Mail account for receiving messages (IMAP/POP/.Mac). To create a new receiving account, use the ‘pop account’, ‘imap account’, and ‘Mac account’ objects
contains mailboxes; contained by application.
account directory (file, r/o) : The directory where the account stores things on disk

I can get the id for the message with the code below but then I’ve hit a wall.

tell application “Mail”

set selectedMessages to selection as list

repeat with thisMessage in selectedMessages
	set theID to id of thisMessage
end repeat

end tell

Thanks for your help.