Find out who registered the account, with the help of search engines. In the search enter the query: "mailbox@domain". In that case, if the address specified by you at least once "lit up" in the Internet system will show you information on who owns email that is identical to your request.
If you still opened a letter from an unknown sender, write to him and ask him to respond. Once you have an answer, you will be able to get the ip address of the computer that was sending the letter and what the city owns this ip address. The list of owners of this email will be significantly reduced.
Go to the forum and create a topic with a question about these letters. Maybe someone already faced with such lists and knows who is registered in the account of the postal system.
Visit various social networks, where you can find the information you need about the owner of a certain email address. If the letter came from the service try a search of the owner of the mailbox in a social network "My world". Enter in a line of search of the necessary address and click "Find". If e-mail is authentic, and its owner was in "My world", you will receive information about the owner'. Still unable to find the owner of email in the social network if the letter service in the same way look for the owner of the account in "Vkontakte", "Classmates", etc.
Go to the website Find something you address in this system. She finds the information in several search engines, and then outputs it in a single consolidated list.
Take advantage of a network of special services that provide information about the owners of the email boxes at your request. Enter in a search engine request.