Ommail sender address 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 firstname.lastname@example.org) 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.