Have a look at the database.
If there are no entrys in egw_accounts, (not even Groups (account_type=‘g’))
or only entrys with
user_type=‘u’ there is no way, this will work.
Set your way of Authentication to SQL.
Create an Admin User.
This will Create the User, the Admins Group and the Default Group.
Login as that Admin, Check if there are any users with Admin->User accounts.
If there are Users (your Mail Users), check Group Permissions. If no Group
Assignment is given.
Assign Groups as desired. Do not forget to assign Admins Group to one of
your existing Users.
Go Back to Setup, switch back to Mail, see what happens.
If there are none of the Users you defined with your Mailserver, your
Authentication with the
Mailserver may have failed entirely.
Note that you may have to login with the emailaddress to authenticate with
your Mailserver.
Note that your domain defined in header.inc.php may have to match your
Maildomain.