Caught in Spam Filters

Greetings
New to "running" the company email system, we swpped over from verizon to our own domain name, vanzyverden.net. Lately a lot of my e-mails end up in customer spam filters or not delivered at all. Even when I respond to a customer's email, which would indicate that I'm on a safe list in there system, the e-mails get lost or caught in a filter. Lately it appears to be getting worse. I have actually lost jobs because customers or prospects don't think I'm interested because they don't see a reply. I have scanned our address through Spamhaus and MX tools, and I'm not sure if I'm doing it correctly; but we don't appear to show up on any block lists.

What tweaks can I do to make us more acceptable and get rejected or filtered less?

Thanks
Clinton