Assuming a person gets his own domain address, say, theperson.com.
He then goes on to create a vanity email address with that domain. e.g. firstname.lastname@example.org.
email@example.com may be hosted by the person's own mail server, or somewhere else that offers vanity email services, such as Gmail, Yahoo mail etc.
To the uninitiated, using a commercial email service is useful as the service is already set up and running, and provides a more secure and stable environment compared to setting up a personal mail server from scratch.
So for an outsider, how do we find out which email service a person is using? Can I find out that firstname.lastname@example.org is actually hosted by Yahoo, or some other service?