This could mean that:
1. The email being sent is too short or looks like spam.
2. SPF and DKIM are not enabled or setup incorrectly - This can be enabled or checked in the control panel of the domain.
3. The servers IP may be blocked which can be checked by putting the domain in the search box of: https://mxtoolbox.com/blacklists.aspx
If it is blacklisted then we may have to request it to be unblocked. Kindly inform me to escalate you to an agent and inform them to unblock the IP if possible.
Here is a powerful tip: Go to https://www.mail-tester.com/ and try sending the same email to the email address they give you. A report will be generated giving you a real view of where the issue could be.
