Hi:
The issue is that C4C supports the SAML name id format "unspecified". Where as if you map the email address (or other attributes) Azure AD sends the name id format as "persistent" / "email" etc (which is not supported by C4C).
So on the Azure AD side the Attribute Name, nameidentifier, has to be mapped to the special function ExtractEmailPrefix(). With this Azure sends the email prefix as the name identifier (as Mattias outlined above) and the name id format as "unspecified". This works with C4C.
Best regards
Venki