Most of my clients are businesses (offices) and they send a lot of emails and a lot of contracts, specification-sheets, and other PDF attachments. They also deal with a lot of real estate agents, so there are often photos or logos in their auto-signature.

I use cPanel to create email addresses for the clients who need them -- personally I prefer the stubborn clients to want to keep their Hotmail and just want an email forwarder hooked up (saves space) but a lot of them are excited to have a professional email address with their business's domain on the end.

Thing is, I'm getting more and more clients experiencing bounced emails within the first couple of weeks of getting their email address. I know for a fact these people are not sending spam (my clients are pretty much only those who work with my boss, and I work in a small office that manages the production of new homes). One such client is getting 1+ bounce notices a day.

How do I fix this? And how do I prevent it from happening again? Do I have to just make all of my clients get a Microsoft Exchange email account or something? Because that seems like a waste and a lot of my clients won't go for that.