Typically when you see this error you should see something listed in the Application log of the Windows Event Viewer - that would be the best place to check.
While you appear to be successfully receiving your emails, here’s another thing to think about: if you've configured FPSE to sent email results to a remote SMTP server, I would suggest configuring the SMTP service on your server as a relay to the remote SMTP server, then configure FPSE to send emails to localhost as the SMTP server. Here's why: FPSE tries its best to communicate with whatever SMTP service is configured. If it runs into any problems, including a momentary slow link, you’ll get errors from FPSE on the form submission. However, if FPSE sends to localhost for SMTP then it should always succeed, and the local SMTP service now has the burden of relaying the information to the remote SMTP service, but it has great features like retries, authentication, etc. In short, the local SMTP service becomes something of a message queue server for FPSE/SMTP.