Reverse Lookup An Email Address
You can always see what happens when you use that old email address to try to send them a message. What can happen after that depends on a few things. For one, the email may never reach the person if they no longer use that address. They may also not open the mail because they do not know your address, or it might be filtered into their spam box. If any of those things happen you won´t hear from them, and you will have no idea what to do next, if anything at all.
You may feel better about sending an email if you are sure they are still using it. That is when you can try an email search online to see if you can find a match. This can be hard, but it does happen for many when they go to the search engines. There are also some people search sites like Bigfoot.com that can help you reverse search an email address for more information.
Even if someone is no longer using an email, it could still be out there. You can search both social and professional networks with that address to see if a profile match comes up. Though they may not longer use that email much, they could have used it to sign up for their account. If you get a match, you can then go to their profile to contact them if you wish.
When it seems that there aren´t any online sources that are going to help you with your email trace, you have one more option of which you may not have considered. You can use some of the same sites that you use to trace phone numbers to try to find out more about any email address.