Well if in your environment you are accepting mail from unknown and untrusted users, it will result in trouble!
Users should be responsible for their own SMTP service really, and most will authenticate to their own email server to send mail.
If you still want to route the mail for the hotspot users only, then the open relay option is the one to use, but restrict access to accept mail ONLY from your hotspot users, and not from the outside world. Perhaps IP restrictions on the SMTP server. The risk here is from any spam bots that may be on the hotspot users machines - this will automatically use your open relay if you are redirecting all SMTP traffic through your system.
Authsmtp is a service where you set up accounts for people, they then use the username and password to send through their servers, but this is probably not suitable in your enviornment due to the nature of the users coming and going.
Shaun
