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Just FYI,
I 'll repeat what I said on comment #6. I did try doing as you requested by setting the "Allow sending email from any address" and it still fails to work.
I backed down the ZCO version to 5.0.15 and it is now working.
It will work. Which user did you set Allow sending from any address on? You should set it on the delegate -- that is the user who is sending on behalf of.
That being said -- we plan to address this. Hopefully soon we will fix this so you won't have to set that preference anyway.