If an invalid email address is entered, let the user know straight away so they can correct any errors.Use the unique Email-Checker j Query version to inform the client of issues with their email address whilst they’re still completing the form.
If the addresses don't match, contact the person you sent the original message to and ask them if they have an email rule to forward messages and ask if they know it's working correctly.
Their rule could have tried to send a copy of your message to a bad email address.
The "show details" button next to the sender details only gives a subset of information, not the full headers.
I tested sending a message from the gmail interface and found there was no X-Originating-IP or any header which gives any sender IP details. Note that the X-* headers are optional headers and not required by the SMTP protocol.
They are usually pretty good at shutting down accounts of anyone who is abusing the service. Select the "More options arrow" here -- Select "Show Original", a new window will open with a full HTML page including "sender" and "recipient" information. :) And of coarse I've clipped the private information but you will see a full page when you execute this function. Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).