Server triggered email issue

Our website (fundingaltdev.co.uk) is hosted on AWS. The domain name is held at our DNS (we are not using Route53) and we are sending emails from yet another a remote email server (@fundingalternative.co.uk addresses as provided by https://theemailshop.co.uk/). We have configured our MX records, etc to enable the web server to trigger emails from the email server, however, we have noticed that while we do get emails delivered to social addresses like yahoo, gmail and hotmail accounts, emails don’t get sent (or received?) to certain email addresses, especially emails which are addresses to our own email server, i.e.: @fundingalternative.co.uk addresses.

The email service provider has said that the Cpanel account is rejecting mail from amazonaws host due to the sender verification failure, because emails are being sent from a separate email server, Citing this reference https://support.cpanel.net/hc/en-us/articles/360052376934-Amazon-SES-Email-address-is-not-verified-

A test page was configured: https://fundingaltdev.co.uk/email-test/ and as per the logs:

curl -I http://fundingaltdev.co.uk/email-test/
HTTP/1.1 200 OK
Date: Thu, 01 Jul 2021 17:11:58 GMT
Server: Apache/2.4.41 (Ubuntu)
Link: <https://fundingaltdev.co.uk/wp-json/>; rel="https://api.w.org/"
Link: <https://fundingaltdev.co.uk/wp-json/wp/v2/pages/472>; rel="alternate"; type="application/json"
Link: <https://fundingaltdev.co.uk/?p=472>; rel=shortlink
Content-Type: text/html; charset=UTF-8