Ok, going by the code:

// get source code of a specific message and cache it function rcmail_message_source($uid)

This code checks if its been cached, and if not pulls the headers and then checks if there is a messageID in the header. It does this from the cached copy if it already exists.

if (!$cached) $headers = $IMAP->get_headers($uid); else $headers = &$cached;

$message_id = $headers->messageID;

I'm not sure how the sessionID is not unique enough. If its good enough to keep a concurrent session active, why is it not good enough for this purpose? Or, do a hash of the time, or something similar.

I think the extra step here would be to somehow write the message_ID we create back to the cache. That way you guarantee each message cache has a unique identifier.