This guide refers to email bounce back messages.

You can determine the severity by the failure on the first number for the status code.

Status: 5.X.X -> means that it was a soft bounce and that you should try resend
Status: 4.X.X -> means that it is a hard bounce where it could be a nonexistent email or mail server saying no, and they will need to get in touch with the person to correct this

This email has not been delivered to the recipient Full details from recipients mail service:
Action: failed
Final-Recipient: rfc822;
Diagnostic-Code: smtp; 550 5.5.0 Requested action not taken: mailbox unavailable. []
Status: 5.5.0
Diagnostic-Code: smtp; 550 5.5.0 Requested action not taken

If this is not enough, refer to the below when dealing with queries on servers and how emailing works:

The IPv4 address for ezyVet is dynamic
Our server's are hosted on the amazon server so the IPv4 address is handled by Amaznon.

We use amazon SES and send to their end point, where amazon then handle all communication from there.

Sometimes email communcation may be bounced due to blacklisting -  As some people send malicious content through SES, and we are all sharing these servers, when amazon kills the server and creates a new instance, this creates a new IPv4 address making it rolling.

Our team recommend the IT person might want to refer to the amazon SES documentation to handle when the Ipv4 address changes to improve this further if needed.