You want to be extremely careful letting people post HTML pages under your own domain, because that can expose you to XSS and such nasty things.
For exemple, most browsers will protect cookies so that one domain can only see cookies that were sent by itself, but will let everything from one domain happily read all cookies sent by the domain indiscriminately. If you let soomeone do a simple html page under the draconity.org domain, they may add a javascript to it that will gather your authentication cookies and send them somewhere... Then they encourage you to visit it and poof, they get access to your admin session and can do everything using your credentials (I can search for proof of concept code for you, if you wish).
Also, if you want to go with it anyway, make sure that people have credentials for uploading that they can only use over ftps, sftp or https (basically, make sure they're always encrypted). You don't want such credentials to go out in cleartext on the network.
Regarding php, it isn't much of an issue, but you should be aware that php has a number of "dangerous" functions that can be used to compromisse the server or other sites running on it. Be sure to check what extensions you are using and disable any function that is dangerous in the php.ini file (and generally, check for any weakness there). Look around on the web for what php functionality are considered dangerous, there are many misleading things about php.
Regarding mail, if you already have a mail server running and that it seems to be working fine for incoming mail, adding email addresses shouldn't expose you to trouble. You don't open the service for sending mails - no one needs that and it's making you risk to get your server used by spammers. If you set up a webmail service, it would of course be an exception - just make sure it's secure.