I have a few clients in need of a SMTP relay as their ISP does not provide one. These clients are in the hospitality business and will be offering this SMTP relay to their guests. Now as this will be used by the public, I can't guarantee that their computers won't be comprised by a virus/trojan/worm.
I'm looking at setting up Exim on an LEB with one IP address and using iptables to only allow traffic over port 25 from a few whitelisted ip addresses.
How can I protect my server from spammers and avoid being blacklisted? Is it just a matter of limiting the rate of emails per hour?