Unfortunately, it is difficult to tell exactly what could be causing your form flow email step to fail from the information provided. Depending on how the email task is failing, there are 2 different places in which you may be able to find logs with more detailed information. The first place is within the Sitefinity error logs that are located at the following location: <Site>/App_Data/Sitefinity/Logs/Error.log. There may also be information in the TPC Error Logs. In TPC 5.3 or below, you can find the TPC Error Logs by navigating to Administration > Portal Connector Configuration. Under The Portal Connector, click on the Error Handling tab, and then select View Error Logs. If you are on TPC 6.0 or above, navigate to Administration > The Portal Connector > Configuration and the under the Error Handling tab, click View Error Logs.
In the case that you are unable to locate any useful information, please reach out to firstname.lastname@example.org and we can help investigate further!