The issue is now resolved.
At approx 8:03am CEST we had an issue with our API which impacted message delivery.
Our Engineering Team was alerted and immediately launched an investigation.
At 10:17am CEST a solution was deployed and the messaging service was restored.
By 10:46am CEST the messaging service had completely stabilized.
We have rolled back to a stable API version and are taking steps internally to prevent a reoccurrence.