Trying a test send on any email gives an exception.  I've seen this error on old instances.  This public preview was provisioned in the last week.  We have also applied the update that was available (1st april release) from 29/3/2018.

Instance  - https://kcl.crm4.dynamics.com/main.aspx#168321668

Error:

Exception 'InvalidPluginExecutionException' occured: Failed to pre update email test send., CorrelationId: 100ed91e-a6c1-4c94-ad0a-e66a0ee6125b