After creating username mapping under samba, "valid users" option is NOT working as expected

Issue

• After creating username mapping under samba using "username map", "valid users" option is NOT working as expected
• If "valid users" consists of AD user which is mapped to an UNIX user, samba share is NOT accessible:

# cat /etc/samba/users.map 
testuser = GSSLAB\aduser
...
# cat /etc/samba/smb.conf | grep "valid users"
valid users = GSSLAB\aduser
...
# smbclient //`hostname`/testshare -U GSSLAB\\aduser%RedHat1!
Domain=[GSSLAB1] OS=[Unix] Server=[Samba 3.6.23-35.el6_8]
tree connect failed: NT_STATUS_ACCESS_DENIED       <--------- Not Expected