We have been experiencing a similar issue sporadically when trying to send email to accounts on smtp servers at gmail.com, ruggedinbox.com, and riseup.net from the smtp server at onionmail.info
We don't believe it is an issue with client configuration or a claws bug. We believe it is a structural issue that arises from poor interaction between the CA model of public-key infrastructure and the asynchronous nature of email. You see, unlike the world wide web, email protocols do not require both servers to be online at the same time. Emails can be stored and forwarded by intermediate servers and, as a result, most email servers have not traditionally enforced any kind of encryption to other servers (since they may have to trust a man-in-the-middle server anyway). Fast forward 20 years and, now that most of the email servers people use are online 24-7 so that end-to-end encryption can almost always be used between servers, many email servers seem to being playing games with their DNS records and public-keys as an anti-spam measure.
Now, in the face of mass surveillence, consciencious administrators are setting up their smtp servers to require encryption instead of defaulting to plain text, but this means that they're having connection failures when trying to connect to traditionally configured servers and/or servers playing anti-spam games with the public-key infrastructure (or also, possibly, servers experiencing real man-in-the-middle attacks on their encrypted connections).
If we are wrong and it is a client configuration problem or a claws bug then, please, fix our code: