Microsoft provide the ability to use their own DNS service for you new Office 365 service.
Simply point your Domain Name Server to resolve to ns1.bdm.microsoftonline.com
In the Microsoft Online Portal - ensure you add in the entries for the non-Office 365 services that your domain may still need; items such as www resolving to another hosted address or vpn to your own site;
Add a CNAME entry for mail.domainname.com
to resolve to mail.office365.com
Also note that http://domainname.com
will not resolve to your homesite unless you add in an "A" rec for @ to resolve to the location you want.