Sophos Data Control - Data Loss Prevention Policy

Have you narrowed it down to specific rules? 

If you disable all the rules but leave data control enabled in policy does it work?

If you run Process Monitor when launching Outlook, do you see any access denied entries that data control could be blocking?

The other thing might be to enable SSPService trace logging by first disabling Tamper Protection, then setting:

[HKEY_LOCAL_MACHINE\SOFTWARE\Sophos\EndpointDefense\LocalConfiguration]
"LogLevel"=dword:00000000

Restart "Sophos System Protection Service" service.

Reproduce the issue then check the log \Programdata\Sophos\Endpoint Defense\Logs\SSP.log.

Filter for all lines (Notepad++ is good for this) that contain [DLP].  Does that help in any way?

Don't forget to stop the service, remove the LogLevel DWORD and restart the service as the log is pretty verbose!

I can confirm if I enable DLP but don't set a rule Outlook opens fine.  If I add a canned Sophos content rule to monitor banking details and Outlook, then Outlook will not open.  I ran through the Proc Mon stuff with support, they sent a few requests which I have completed by adding in exclusions or another one was to disable all Outlook add-ins, which is still in place (inactive) and I still have the issue.  

Starting with Outlook closed, did the SSP trace log help indicate anything when it was launched and it failed?

I am not able to access the log file at this time as it required elevated credentials.  I did re-run a proc mon and reviewed it and don't see anything sticking out with a failure or error - a lot of successes.