Hey @Hannah, you get this error when your elastic IP has created an rDNS(reverse DNS). Execute the following command to find out if an rDNS exists:
if it exists then you'll be shown something like this:
203.0.113.0.in-addr.arpa. domain-name-pointer mail.domain.com
Now you need to remove that to get rid of the error. Fill the Request to Remove Email Sending Limitations form. Now try releasing the elastic IP.