The response might tell you what Aunt Mabel's address is, or it might explain how to figure it out. Many companies have a policy on how they assign addresses. Bigger companies are more likely to require that all email addresses follow one of the above schemes. However, the bigger a company, the more likely that extra characters may be inserted in order to make the address unique. If Aunt Mabel is a student, teacher, or staff member at a university, you may be in luck.
Most universities have some sort of email directory.
For example, see the University of Illinois' online phone book. To find the phone book, find the university's web site and then hunt around a little bit. Usually, universities list their directories or phone books on their top page. If you can't find their directory, you can guess at the email ID, using the same kinds of rules as above. Universities, since they serve so many people, are likely to run out of simple email IDs.