Do we need to add cname record for autodiscover in office 365

I have an o365 tenant that have a exchange server 2k13 on-premises with hybrid configurated to o365 and using SfB online only.

in public dns I have A record point autodiscover.domain.com to IP of CAS server.

My question is, do I need to add a cname record autodiscover.domain.com to autodiscover.outlook.com, and WHY?

What happen when I didn't add the cname record to public DNS?

 current situation:

- all my users using SfB on PC are working normaly but not for SfB on mobile.

- for SfB mobile, a user cannot get the metting information from exchange server ( the user's mailbox is on-premises)

when I check the response of autodiscover I saw that the CAS server return error code 401 for the request:

 GET https://autodiscover.domain.com/autodiscover/autodiscover.xml

Request Id: xxxxxx
HttpHeader:Content-Length 0
HttpHeader:Date Mon, .....
HttpHeader:request-id .....
HttpHeader:Server Microsoft-IIS/8.0 Microsoft-HTTPAPI/2.0
HttpHeader:StatusCode 401
HttpHeader:Www-Authenticate Negotiate, NTLM, Basic realm="CASservername.domain.com", Bearer client_id="0000000x-0000-xxxx-xxxx-000000000000", trusted_issuers="0000000x-0000-0000-c000-000000000000@xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxx", authorization_uri="https://login.windows.net/common/oauth2/authorize"
HttpHeader:X-FEServer CASservername
HttpHeader:X-Powered-By ASP.NET