Other contributors to emails getting marked as spam

[maestroc]

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?

 

[cPanelLauren]

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.

IP reputation plays a big part of this, you can check your current IP reputation with services like sender score

What steps can I take to check for configuration issues?

Generally speaking the steps outlined in the following documentation should be followed: How to Keep Your Email Out of the Spam Folder | cPanel & WHM Documentation

How should the server be configured for reverse DNS if they are all on a shared IP? Is this even a potential problem?

rDNS should resolved back to the hostname of the server, so no this shouldn't be an issue regardless of how many accounts are sharing the IP.