Is there any way to test the email function to get a better idea of why this isn't working?
Here's the message I'm getting from the user side:
RSVP successful. However, there were some problems whilst sending confirmation emails to you and/or the event contact person. You may want to contact them directly and letting them know of this error.
Plain text emails.
Notification email address is the correct one for the account.
Port is 465.
Mail sending method is SMTP.
Use SMTP authentication: yes.
Host: smtp.gmail.com (also tried ssl://smtp.gmail.com:465)
Username: full google apps firstname.lastname@example.org
Password is correct.
Not sure where it's going wrong. This same account works fine through WP-Mail-SMTP for other transactional emails on the site.