From Oauth module you get an email X. But this email X is not included as account in your Mx DB.
1) Import the accounts first or in the resolveuserbyemail from your IDP ( you can use LDAP/SAML) and you have the account with email to resolve against.
2) But seen you already have authenticated with your IDP, you are sure that the user should get access in your Mx, so why not make an account if there is not?