What happens when you instead use the FQDN of the target Samba Server?
smbclient -L your-server.example.com -U username
substitute the your-server.example.com for the real fqdn of the system (e.g. samba-server.corp.contoso.com) - the system name should be resolvable by DNS for proper communication.
Did you follow the instructions (outlined in the manual or the blog I linked to) on how to set up a test share?
What is the output of adinfo --diag in the system hosting samba?