i will attempt to reproduce the bug here. 
may be there is a specific problem only with Yandex Mail (SMTP) but i don't test it with other mail providers.
this is a test account of Yandex Mail. try it please to reproduce the bug (emails will not send without my patch):
url: https://mail.yandex.com/
login: smtp-test@modulez.ru
pass: 3z2eR5ksyaGqLg9JIRD5