This issue occurs if all the following conditions are true:
- You use a custom domain in Office 365.
- DNS SRV resource records, CNAME resource records, or both kinds of records are configured incorrectly for your domain.
Troubleshoot Skype for Business Online DNS issues with the Lync Remote Connectivity Analyzer (RCA)
Go to the Lync Connectivity Analyzer and select Office 365 Custom/Vanity Domain Name Settings Test for Lync. Enter the sign-in address that you're using when you to try to sign in to Skype for Business Online (for example, email@example.com), and start the test.
This test examines all four DNS records that are required by Skype for Business Online to determine whether they're correctly configured.
If any issues are detected, check the DNS configuration with your domain registrar. For instructions about how to configure DNS for the most common DNS providers, go to the following Microsoft Office 365 website:
If you can't add SRV records through your DNS host
Some DNS hosts can't host SRV records, or the DNS hosts don't have options to host SRV records. This condition blocks the ability to communicate with external users (Federated and PIC). In these cases, there is currently no workaround except to change DNS hosts or to manage DNS through an on-premises DNS host.
Some UNIX or Linux OS servers that host DNS don’t require the underscore (_) when you add SRV records to the DNS host. If your DNS host is running UNIX or Linux and the SRV records aren’t resolving correctly, remove the underscore from the host name of the SRV record.