I'm having an issue with this too.
I installed a few months a version of this plugin in a site, which I modified a little bit for it to accept more file formats than just images.
Anyway, everything worked fine with that installation, but I just installed a new WP version and this plugin from the wp plugin search, and as I make tests I can't get it to send emails both to the participant who registers nor to the "Signup Notification Recipient".
The item is saved to the database, its just the email which is not sent. I checked settings and fields, and I have no clue as to where to look next. I checked spam folders, the settings on the plugin, the actual fields of the database.. everything is the same or similar from the last time I used this plugin...