We use Microsoft Graph to subscribe to webhooks from emails. Additionally, as a backup procedure we also crawl the messages directly.
We crawl around 5 million emails a day, and we see that each day consistently around 1%-2% of these emails are not sent to us via the webhook, although the subscription for this principal is active (and other email notifications from this user are indeed sent).
Is there any logic on the Microsoft Graph side to not send webhooks for certain types of emails by design? or is it just a problem on the notification mechanism?
(Obviously crawling them, as we do now, is a viable backup option, but that also means the processing of the email will be delayed)