mrobben,
Are you saying you have two UE instances on the same page, one sends and the other doesn't?
If this is the case, make sure the first one does not redirect to another page before the second one can trigger.
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takingchances,
If the form send to some email addresses, but not others, this will be a problem with the smtp server, or the messages are being caught as spam. The smtp server is not relaying to some addresses, but will relay top others, you should contact the host to see why it is not relaying to some addresses/