Other contributors to emails getting marked as spam

If we assume that the content of the email is not spammy what else will make a domain's mail wind up going to the recipient's spam folder? In this particular case I have two hosting clients claiming that their emails wind up in spam unless the user specifically whitelists them. Other details: Most of my clients are on a shared IP address however the IP address, the domain names, and the server itself do not appear on any blacklists. What steps can I take to check for configuration issues? How should the server be configured for reverse DNS if they are all on a shared IP? Is this even a potential problem?