Sender for mail destination is missing.
Ommail needs to know the sender address to deliver mail. This is required to generate wellformed smtp messages. Relay servers will often ignore messages without a sender. It was tried to create an ommail action for which no sender address was specified. Note that it may not required to provide a real sender address, a dummy address (like email@example.com) might be sufficient.
The most probable case is that some configuration statements are missing from the action definition. See ommail documentation for a configuration samples and the required configuration statements.
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