If you are using emailpassword / thirdpartyemailpassword / thirdparty: In case that doesn't work for you, and you actually want to use multiple domains, then you can set this value to be anything you'd like (doesn't matter), as long as you override these functions (in the recipe init config): -

resetPasswordUsingTokenFeature

->

getResetPasswordURL

function -

emailVerificationFeature

->

getEmailVerificationURL

function --------------- If you are only using sessions, then you can set this value to be anything..