Lync constantly prompts users for username and password

Lync constantly prompts users for username and password even though they check the box  to save password.

I typically see this issue when a customer’s SMTP address is different than their SIP address and there is no domain for the SMTP address. This can occur because Lync expects NTLM to be the provided authentication. You can force IIS to use NTLM, which should resolve the authentication prompts.