takayukister, your plugin is the only thing that changed on my site. Prior to the latest update, things were working just fine, and subsequent to that last update, this problem arose. I haven't made any other changes to the infrastructure that would've caused such a problem, and certainly not to any of the e-mail settings within the site.
Your plugin has to pass the sender's e-mail variable to the mail server, does it not? Could there be some kind of issue in the way that is being handled? It seems to me as though the variable is being sent as the recipient's e-mail address rather than the sender, and if that's the case, then I'd suspect something amiss in the plugin itself.