System email messages are sent through Sendgrid servers. The Sendgrid servers have special message headers they add to try to help with bounced messages. However, there are a lot of factors that affect bounced emails, and different mail servers handle them in different ways. Many bounced emails tend to get swallowed by systems or end up flagged as spam.
Reference requests are set to be bounced back to the applicant.
Reviewer Invitation/Assignment Notification
Reviewer invitations are set to be bounced back to the administrator who invited them.
There is no definite way for an administrator to know if an offer notification email to an applicant was bounced back. One suggestion would be to setup a forwarding rule on the site email address to forward all emails sent to the site email address to administrators, but there is no guarantee that all bounced offer emails will bounce all the way back to the administrators correctly.
The system does not validate the email that is entered in the Site Email field (Site > Settings > Content); therefore, clients must configure a working email to be entered in this field. If the site email address is not valid and anyone attempts to reply to an email from it, the person who attempted to reply will receive a bounce back email.