Thanks for reaching out. A transactional email service is something like Mailgun or Sendgrid, among others. It is usually set up via an SMTP plugin or even a plugin provided for that specific server (like the mailgun plugin, written by Mailgun itself). In that client’s particular case, the service was modifying the reply-to header, making it all lowercase. That broke the decoding used to identify user, forum, etc.
The best thing you could do is to look at the headers in one of the emails sent to the RBE inbox and check if the Reply-to address is all lowercase. If it is, then that’s definitely the issue.
If it’s not, then there are other troubleshooting steps you can take. For example, make sure your RBE is set to use the webhook and then copy/paste the webhook address in a new browser tab or window. Let me know what it says. It also helps to know more about your setup, like if you’re using Gmail for RBE or some other service, and if you are indeed using a transactional email service to send out notifications from your site.