The Email Queue’s design was such that it was single-threaded per client, and as the number of clients has grown, it has become more likely that one client (or many clients on a busy day like the beginning of the month) can slow down processing for others.

In order to address this, we redesigned & segmented this service into our latest design which uses low & high priority queues based on volume and prevents clients from slowing processing for each other.

This new version has completed testing and has been deployed and is live into production, and we are confident this resolves the issue going forward.