We use a state of the art email delivery infrastructure designed for "transactional" email (not an email marketing system). The vast majority of users will not have issues receiving the notifications our systems send.
Because of the numerous factors involved in successfully delivered emails such as spam and junk folders, company and organization filters and more, we cannot guarantee that every email will be delivered. Some may go to the "junk" or "spam" or "promotions" folder or get "stuck" in organization level filters.
Unfortunately, these situations are beyond our control and we cannot guarantee successful delivery or offer credits for non-deliverable emails (our service still charges us whether or not the email is successfully delivered or not). We also do not monitor if emails are delivered, opened or bounced.
Keep in mind that if a user continues to have issues sending a notification to the intended recipient, often forwarding the original gift card notification from Shopify can be more reliable.
We also provide the full URL to the web version fo the gift card on the scheduling page so customers can copy and paste that link into a new email that they personally address to the recipient, which is often more successful.