Proper MX Record

Our MX record has several entries. Some of them are completely correct and pass every test at mxtoolbox.com, but some have no A record, which are for our edge servers. We kept them in there because that was what was done in the past. We now use Zscaler for our outbound email.  I believe the edge servers were accurate before we used Zscaler and they was the entry point of email for the outside world. Now these edge servers only send and receive email to/from the Zscaler servers.
We received a few Delivery Status Notifications with the errors in the attached file.
My questions are:
•      Having entries in my MX record the reason for this email being rejected?
•      Are having entries in my MS record without A records not RFC compliant?
•      We have no such errors on 99.99% of all our outgoing email, what would be a good reason for this domain to be configured to be so sensitive to an MX record having some entries without A records when it also has other entries that do have A records.
•      With the above information I provided, can you see any value in leaving these edge servers without A records in our MX record?

Thank you,
Delivery-Failure.jpg