samba4 refuses to auth one user

[Pavel Herrmann]

Hi,

I have a little problem with my samba4 (which is otherwise great, good job 
guys!), it refuses to authenticate one of my users (yes the password is 
correct, tried resetting it)

the users name is domname at domname.tld - basically it works as a shared mailbox 
for staff, not as a real user.

is this some special account name which cannot be used? or where else can be 
the problem?

Thanks