A possible reason for getting this error is if the email address being used on the donation has incorrect characters in the email address.  For example test&donation@blackbaud.com.  The ampersand and the period at the end of the address are not recognizable by BBMS, thus the merchant account will reject the transaction.

Note the example above is just one possible scenario.  This can occur with any fields that include unusual characters in the submission (phone numbers, email, name). Small set of invalid symbol examples:  %,$,&.  
 

Contact Luminate Online support if your transaction does not seem to be impacted by the issue above and it can be investigated.