gawd ... I hate to even think this out loud... but... seeing as search engine friendliness is not really any issue for those pages, have you considered wrapping the mailmail pages in an (ewww!) iframe (for easy look and feel integration) and storing them on a subdomain that isn't impacted by the WordPress .htaccess rewrite rules?
Or,for that matter, you could stick those pages in their own subdirectory within your site and stick some rewrite rules into your WordPress .htaccess to allow for them to be "reachable" using the workaround for the DreamHost default "stats" subdirectory that is in the wiki.
Or did I just make it even more confusing?