That' difficult. RS offers you the option to have forgotten usernames sent to your email but it doesn't offer you a way to recover a forgotten email address formally.
If you go to the RS Support Centre and go under 'Account Help' you can pick what sort of issue you're dealing with. They'll give you guidelines on how to work out any issues you have and if your problem can't be solved by following the instructions they've advised you to take than they suggest you click the 'Contact Us' button at the bottom of all the writing. It will direct you to an information page where they get you to write down any information you remember regarding your account. This inclues information such as emails, your IP adress (do NOT use a VPN, make sure you have your home IP address so that they can verify it's you and that it matches the IP you used to register the account), your recovery questions, your payment information and additional information which you can use to write a short letter explaining your predicament.
If the RS system doesn't work, I'd advise contacting the email company you used and asking them to track any and all accounts registered under your IP. This way they can tell you what your forgotten email account address is and you can simply log into your RS account.
Good luck, all the best with your account recovery.
