If your Outlook client machines are still trying to connect to your old onsite exchange server even though they are connected to your new Offcie365 service AND they are on an Active Directory domain (obviously), this might be why:

Outlook uses SCP (Service Connection Point) to autodiscover your local exchange server before it tries DNS, so it’ll still find the old one even if you have the correct DNS autodiscover set up. You can disable this in the registry:

Add: ExcludeScpLookup=dword:1
(This is for Outlook 2010, Outlook 2013 and Outlook 2007 will be the different area such as xxx\15.0\xxxx)

Thanks to Jason Jiang for the knowledge here

You can also delete the SCP record altogether using ASDI edit:

  1. Select the “Configuration” naming context
  2. Drill down to:
    CN=Microsoft Exchange\
    CN=<Your Org name>\
    CN=Administrative Groups\
    CN=Exchange Administrative Groups\
    CN=<Your Server Name>\
  3. And delete the CN=<Your Server Name> of class serviceConnectionPoint in there