A Microsoft reference is here:

http://support.microsoft.com/kb/940881

with further notes here:

http://www.thirdtier.net/2009/02/setting-up-an-external-autodiscover-record-for-sbs-2008/

This has to be set at the Internet level only. We’ll use “domain.com” as the domain name, just replace it with yours.

1. If there are any A or CNAME records pointing to “autodiscover.domain.com”, delete them. If the cert for your server uses “autodiscover.domain.com”, change it to something else, e.g., “remote.domain.com”.
2. If there are any A or CNAME records pointing to “*.domain.com”, an asterisk for global, you’ll have to remove them too. You will need to replace with multiple specific records, or a subdomain, if you’re using a global configuration.
3. In your Internet DNS, create a new SRV record:

At Network Solutions, the “Service” pulldown needs to have “_autodiscover”, the “Protocol” pulldown needs to have “_tcp”, “Priority” and “Weight” need to be zero, the “Port” needs to be 443, and the “Target” needs to be the A record name used in the MX record, e.g., “mail.domain.com”.

At GoDaddy, “Service” needs to contain “_autodiscover”, “Protocol” needs to contain “_tcp”, “Name” needs to contain “@”, “Priority” needs to be 0 (zero), “Weight” needs to be 0 (zero), “Port” needs to be 443, and “Target” needs to be the A record name used in the MX record, e.g., “mail.domain.com”.

You can test the setup with nslookup:

nslookup -querytype=SRV _autodiscover._tcp.domain.com

The result should be approximately thus:

Server:  dns.domain.local
Address:  99.88.77.66

Non-authoritative answer:
_autodiscover._tcp.domain.com        SRV service location:
          priority       = 0
          weight         = 0
          port           = 443
          svr hostname   = mail.domain.com