Bug, error sending email from SMTP

Some newsletters are sent at the moment - http://i.imgur.com/wwUwqaT.png . But one letter does not want to sent - http://i.imgur.com/nDjajiA.png and http://i.imgur.com/DfzQqD2.png . All settings are correct, nothing changed. Periodically popping up error sending mail. I use Mandrill.