Sounds like an issue with smtp then perhaps blocked for too many connections if you're testing too quickly
I'll try the keep SMTP connection alive, see if that helps. Thank you for all your support, I appreciate it.
Okay, keeping it alive doesn't help, seems to fail on the 3rd or 4th test, from then on you need to either click send again or hit refresh in the browser "sending the form data again". Fails after that for at least 10min more (each request requiring the refesh) before it works again without needing a refresh of the 'send data again refresh'. I'll have to add some failure control... processing request notice (takes about 12seconds to send) and a retry button maybe? Will have to seek out examples of these, first time creating a form.