Error SimplesamlphpAttributeException Error in simplesamlphp_auth.module: no valid "mail" attribute set.

Problem/Motivation

We got the test in SimpleSAMLphp test tab working. I can SSO login and session started.

But when I clicked the saml login link(/saml_login), I got following error said,

Drupal\simplesamlphp_auth\Exception\SimplesamlphpAttributeException: Error in simplesamlphp_auth.module: no valid "mail" attribute set. in Drupal\simplesamlphp_auth\Service\SimplesamlphpAuthManager->getAttribute() (line 284 of /simplesamlphp_auth/src/Service/SimplesamlphpAuthManager.php).

Drupal\simplesamlphp_auth\Service\SimplesamlphpAuthManager->getAuthname() (Line: 179)
Drupal\simplesamlphp_auth\Controller\SimplesamlphpAuthController->authenticate()

Steps to reproduce

1. Install SimpleSAMLphp 2.1.1
2. Enable simplesamlphp_auth module 4.0.0
3. Config the module with following settings:
   
   • SimpleSAMLphp attribute to be used as unique identifier for the user: mail
   • SimpleSAMLphp attribute to be used as username for the user: mail
   • SimpleSAMLphp attribute to be used as email address for the user: mail
4. The IDP NameID format is: urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress