[Plugin: WordPress Form Manager] Submission emails not sent

I have a registration form that was working fine. Suddenly, submission data is still being written to the data file, but submission emails are no longer being sent either to the admin or to the submitter. Alarming! Nothing has changed that I am aware of. I use Google Apps for email. I’m using advanced email options. Originally the default wp_mail setting was being used. After noticing that emails weren’t being sent I changed to PHP, but still no email.

http://ww.wp.xz.cn/extend/plugins/wordpress-form-manager/