We recently had a customer get a ISP change and on the same day were moving their DNS to Godaddy.  A few weeks later they noticed a bunch of emails bouncing back with a lot of the email addresses being for Comcast. We weren’t sure if this was a Godaddy,DNS, Exchange, or ISP issue.

It turned out to be the ISP needed to a PTR (pointer record) to the IP address.

This link helped us know that the PTR record is set by the ISP.

https://community.spiceworks.com/topic/1212792-ptr-record-in-godaddy-not-working

Here are some of the other things we had tried.

• Adding the static IP to their SPF record.
• Adding a Reverse Zone Record in the exchange server.
• Turned off Spam Filtering in their email account

Let us know if this helped you…