I believe this is an issue with the configuration dialog. You shouldn't be able to enable "Implict SSL/TLS" without first specifying a SSL certificate. The crash is causes by smtp4dev trying to establish an SSL session without having a configured SSL
If you want to use SSL you need to provide a SSL certificate that your SMTP client will trust - just configure that in the options screen and the crash should stop happening.
I'm opening a work item to fix this properly.