I get "unable to queue the message"
Could you post relevant output from the "Network log" window from Claws Mail? (Found in Tools menu, I think.)
The error message you mentioned only means that your SMTP (sending) server did not accept the e-mail for delivery, nothing more specific. Network log might give more details.
Or that's an IMAP problem. Probable duplicate of #2262.
Does this still happen with new builds?
*** This bug has been marked as a duplicate of bug 2262 ***