When a Helm user receives a Google calendar invite from another person and clicks the "Yes" or "No" link to accept or reject the invitation, the Helm user may receive a Delivery Status Notification (Failure) reporting that the message has been blocked:
Why it happens
Google attempts to send an email to the inviter on the Helm user's behalf (an email from Google's server says that they were email@example.com). As a core security feature, Helm's DMARC policy indicates that all emails must originate from your Helm Server's IP and be signed with keys on your Helm Server. The email server for the recipient of the acceptance or rejection will look up the DMARC policy and reject Google's email because they are attempting to send email on your behalf in an unauthorized manner. Because the acceptance/rejection email has been blocked, the initial sender of the invite will not know there was a response.
What you should do
To respond to Google calendar invitations sent to a Helm account, open the .ics attachment in the invite and accept or reject calendar invites from the calendar app you are using. This will successfully deliver your response.