Sorry, auth_default_realm = example.com

Do you use multidomain setup?

No. I only use one domain only.

I assume you use LDAP authentication.
What did you set for IDFieldName and UIDFieldName?

both is "mail" (without quotes)

What is the content of bindFields?
Do you have an attribute which only contains the username part listed in it ?

bindFields = (mail);

with

bindAsCurrentUser = YES;

and

SOGoForceExternalLoginWithEmail = NO;

You have to add an attribute to bindFields, which only contains the desired login name value.
Example:
bindFields = (mail,cn);

I had added

bindFields = (mail,cn);

but only postmaster (w/o domain part) works. Other accounts are still not accessible w/o the domain part.

Regards,

The cn attribute was only an example, as I do not know, which values are in there for your setup.
Does cn perhaps contain the users full name?
Then the user has to login either with his full name or his email.

So you have to find an attribute, which contains the desired login name, and add this to bindFields instead of the cn example.