Transport command failed - No sender specified in control file (1)

I am gettin this error message in MPro having tried to send a messsage 
to a long mailing list (1500 email addresses).

I created a mailing list and sent a message to it in MPro. I now keep 
getting this message.

Does this mean some people have the message or that no-one will 
receive it?

How do I stop it?

Any suggestions?

Regards,

MArk

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