This is pretty self-explanatory, but Salesforce actually has its own method for calculating whether an email address is 'invalid' or 'valid', and Salesforce has judged that these records fall into the Invalid category.
The error message would be the following:
E2120 - ECE_SForceDataError: We got an error back from SalesForce when trying to update or insert data in SalesForce: Error returned from SFDC on upsert: [fields: Customer_Email__c , message: Customer Email: invalid email address: 1 , statusCode: INVALID_EMAIL_ADDRESS ].
The most common cause is bad data such as : spaces in email addresses e.g. "test@ test.com", multiple @ symbols e.g. "test@test@test.com" or invalid characters such as commas e.g. "test@test,com". Make sure you clean up the email addresses before attempting to Insert / Update them into Salesforce.
One way to fix this within RAPIDI is to update your current mapping and add the ISEMAIL formula and filter out any bad data when the records are transferred from your source system to Salesforce.