SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

Is it possible to access the email address that the user used to signup/sign in with at the session initiation on the backend?

you mean when you override the createNewSession function on the backend?

I can get the userId through the input object

Because I would like to send that email along to another service of mine

right yea. The recipe you use, should have a `getUserById` function which you can call with the userId. The return of that is an object that should contain the email