Issue: When trying to enable a new user for Lync, you receive the following error; both within PowerShell and the Web GUI.

Enable-CsUser : Specified SIP domain is not vaild. Specify a valid SIP domain and then try again.

When executing: Enable-CsUser -Identity "DOMAIN\Jack.Stromberg.Admin" -RegistrarPool "lyncpool.mydomain.local" -SipAddressType "emailaddress" -SipDomain "mydomain.com"

Solution: Most of the time this turns out to be a typo in the -SipDomain parameter or the user's email address set inside of Active Directory.  Once you verify that you do not have any typos in the user's email address/SipDomain parameter, the user should be able to be added successfully.  If neither work, you can specify the SipAddressType to be the user's SamAccountName rather than email address.