Sorry no one has got back to you yet... it's unusual.
I saw this earlier but didn't answer because I don't know what the answer is.!
As you are in a hurry... I will just say that some of these problems seem to be authentication on the server. I don't know about this one, but if you are desperate... you could just look in Mailing List Settings and check if Send as Email address is a valid address on the server that List Mail is on. I think I remember that might be needed.
It's late and I am tired so this could be wrong or nothing to do with it but if you have nothing else to go on you could take a look there anyway.
Good luck. (look through the forum, there are some alternative solutions mentioned.)
By the way: Thought I had better repeat - I really don't have a clue about your error - what I said is just a guess.