Some company's use this domain for services like Lync Online.
If you are using the onmicrosoft domain, there is no way to manage DNS records as they are managed by Microsoft.
And as you know, to get Lync Mobile to work you need to add some DNS records.
( And probably they haven't been updated yet as Lync Mobile does not work )
See previous blogpost : http://blog.msgeneral.nl/2011/12/lync-mobile-and-office-365-how-to-get.html
The thing that does not work is autodiscover, but you can manually enter the discovery URL in your Lync Mobile client.
- Start the Lync mobile client
- Enter your username as: <name>@<domain>.onmicrosoft.com
- Enter your password
- Specify a username in the same format: <name>@<domain>.onmicrosoft.com
- Toggle the “Auto-detect services” & enter the following as the “External Discovery URL” https://meet.lync.com/Autodiscover/autodiscoverservice.svc/Root
Thanks to Ben Lee for the explanation ( source ) :