The error message is 501 EHLO invalid ehlo (localhost). I tried various combinations of configuration parameters, but always encountered the same error. My test machine is an HP Compaq Presario tower. I have multiple partitions on the machine, with several Linux distros installed. On Debian-9.8.0-LXDE, Claws-Mail (version 3.14) is installed and it works fine.
This isn't a bug. And this bug tracker is not a support forum. Please use with the users' mailing list or the IRC channel for support: https://www.claws-mail.org/MLs.php If it works fine on your debian install, set the hostname on your windows installation to the same as your debian installation. (Find it on debian by entering the command "hostname"). Alternately, in Claws Mail, go to your Account Preferences, open the Advanced page, and set a suitable value for 'Domain name'. The SMTP server doesn't like 'localhost' and rejects it.