Thanks for your response.

We aren't seeing any NDR's as the emails are being sent by Quickbooks and not directly by us. We have no way to check in any email logs to determine the reason. Quickbooks log only shows that email was sent. When we check the SPF record for our domain in MXTools we can see that there are too many lookups (11) so we are guessing that whichever sending domain is being used by Quickbooks isn't being cached in the SPF record and therefore our emails are rejected by some of our customers mail systems that use SPF, DKIM and/or DMARC