An error similar to the following is sent to the Route Errors to Email Address:
The attached message was sent to the Email Service address <firstname.lastname@example.org> but could not be processed because the following error occurred:
554 System.DmlException: Insert failed. First exception on row 0; first error: FIELD_INTEGRITY_EXCEPTION, ......: [LookupField]
We recommend using an Optional lookup filter. Note: If your organization has duplicate Accounts, Contacts or Leads, E2CP may be using a randomly selected duplicate that doesn’t satisfy the lookup filter criteria while another duplicate does.
Configuration of Lookup Filters is not supported as a part of product support.
This is by design.
See Salesforce Help & Training for information on configuring Lookup Filters.