Fortigate 100D Blocking allowed email

I have upgraded from 5.2.4 to 5.4.5 a couple of weeks ago.

It has been found though that some emails have been getting blocked that were in our allowed list. 

I have removed the item from the spam filter and re-added but this didn't make any difference.  I have also added in the IP as a mark as clear, but the item still gets blocked.

The reason of blocking is "smtp helo/ehlo domain name DNS check failed.". I know why this is the case, it's because a third party fronts the company for their emails. 

Any idea this is being blocked?