Assume that you deploy Microsoft Lync servers as fully on-premises without any lyncdiscover DNS records, and that you have Azure Active Directory (AAD) federations set. After you install the December 8, 2015 update for Microsoft Lync 2013 (Microsoft Skype for Business), you discover that you can't sign in to the client by using the domain\username format. In this situation, you receive the following error message:
The address type is not valid
Valid address use the format username@domain(firstname.lastname@example.org). Make sure there are no spaces in the address.
Click Back to retype the address.
This issue occurs because if Lync 2013 (Skype for Business) client fails to detect lyncdiscover and lyncdiscoverinternal services. In this case, the new DNS-less discovery (also known as Cloud-based discovery) redirects you to the Microsoft Lync Online Discovery services in Office 365. Then, you're forced-authenticated through the UPN format when you sign in to Lync 2013 (Skype for Business).