In the email's inbox folder, you will see a list of emails (messages) that have come. Click on any and the email message will open. Read, check and see the entire message, if you see any website link there or any website mentioned there, then that is what you are looking for.
Incase you are referring to the domain of an email message, then check the "from" part and example it will be something like james @ abcdefg. com (ignore spaced I entered), then abcdefg .com is what you are looking for.