With so many email providers available on the Internet, it is highly possible for one to forget about an email account for which one had once signed up. There are many reasons why it's beneficial to know what accounts you do have, and to not leave an unused account available on a server online. You may need to find past accounts you've forgotten about if you are suffering from identity theft, or you're worried that you've been missing a number of emails you were once expecting to receive.
Make a list of all business you've worked for and schools you've attended. You may have a work or school email that is still active. Go to the website for the business and school, and then try to locate an email directory for employees. If you don't find a directory, email the the owner of the website to say that you are curious if you still have an active email account with the company or institution.
Consider any forums, blogs or websites for which you have written. You may have been given an email account for one of these websites and not even be aware of it. Go to these sites and log in to your account. For example, if you have a Blogspot account you must have had a Gmail account with which you started your blog.
Try a trial and error method. If you know that you have typically used one or two kinds of usernames, then you should try using that name for various emails. Go to an email provider's website and try searching by common nicknames you may have used in the past. Also, try using variations on your own name.
Go to email websites like Yahoo!, Google and MSN to see if you have an email account with a popular provider that you've forgotten about. Find a link to find email addresses, where you will then be prompted to type in your first and last name. You will be told whether or not that person has an account with the provider. You may then need to answer an old security question, or prove that you are the person who created the account.
Find a reverse email website like EmailFinder.com. These search directories can be helpful for finding a lost email account. Simply type in your first and last name and the state where you reside. You then will have to pay a small fee in order to view all results, so this step should be a last resort.
Keep in mind that many email providers will permanently delete accounts that aren't used or logged into within so many days. Read the rules on the email provider's website to see if maybe your past account has expired.