Destination is not enabled: In the form builder we require that you actively enable every destination you want for every form on the site. If you do not, when people fill out those forms they won't go through to the destination and will only be saved on the Run form submissions page. You can enable destinations by clicking the Enable checkbox to the left of them in the form builder.
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If you're checking the form submissions page to see if submissions went through, and it's blank under the destination column, it means that the destination was not enabled. If the destination is enabled and the submission fails to go through, it will say Failed there.
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You're attempting to submit a duplicate: Some of the destinations will throw back a failure message if you're attempting to add an email or other record that's already in your database there.
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You're submitting obviously fake records: Some destinations will block submissions that appear to be fake, such as using words like Test, using area codes that don't exist, etc. So if your submission fails, try again with less fake seeming data.