I was recently researching companies that were blacklisted and discovered that the domain (irio.com) for IRIO Mobile Marketing was listed on: 22.214.171.124.bl.csma.biz,126.96.36.199.dnsbl.ahbl.org,188.8.131.52.tor.ahbl.org.
There are a number of factors that may have caused this. They may have inherited a bad IP address with their email service, or someone might have marked an email as spam by accident. Switching to a new Email provider will supply their domain with a new IP address. Please keep in mind that some blacklists track the domain and not the IP, but acquiring a new IP address will help.
Your customer's Email service can become a gateway for future cloud services. Supplying a new email service today will make it much easier to capture their cloud revenue in the future.
I'm working on an Up-Sell report for IRIO Mobile Marketing and should be ready shortly at this URL: http://bit.ly/2nNwiBg