Error while preparing to send sharing message (Outlook 2007/2010/2013/2016)

I got the following error when I had migrated a customer from a Small Business Server to our local Hosted Exchange.

Outlook on a domain looks to Active Directory to get the auto discover information. If you run the Outlook connectivity tester on the affected computer, you will see that it returns the local SBS server autodiscover settings, even if you have configured Outlook to use an external provider.

The simplest way to fix this if the SBS server is still in used and Exchange is still installed, is to run the following commands:

Get-ClientAccessServer | Set-ClientAccessServer -AutoDiscoverServiceInternalUri https://mail.externaldomain.com/Autodiscover/Autodiscover.xml

And

get-autodiscovervirtualdirectory | Set-AutodiscoverVirtualDirectory -ExternalUrl https://mail.externaldomain.com/Autodiscover/Autodiscover.xml -InternalUrl https://mail.externaldomain.com/Autodiscover/Autodiscover.xml

This then points the Outlook client to the correct external Autodiscover service.

If you have any questions, please let me know.