The insertion is only done once we have received a response from Google servers that the email has been indeed sent (without bounce or other errors).
Also, won't Google servers return a cryptographically verifiable delivery confirmation? If so, couldn't this be used as the delivery proof directly instead?
We only receive response statuses and act accordingly. Moreover, we think it's important for this functionality to make sense to insert the hash corresponding to the integrity of the email, and so make possible for our customers to "prove" they have indeed sent an email with a specific body/subject/...
In other words, the delivery proof is not based on trusting the blockchain mechanics, its based on trusting Gmelius Ltd.
This hybrid process implies you will need to trust parties at some points, e.g., Google and Gmelius. However, the point here is to offer a means to store in a robust way the trace that a specific email communication existed at a point in time, including its context/content.