An issue I noticed looking at the disk io usage graphs.
From the last few days I'm getting spikes close to 100%.

Looking at the queue, theres's 1500 deferred emails for 2 users due to being over quota.

If I delete those 1500 emails, my IO load drops back to normal ~20%
And two days later, the deferred queue has accumulated the same amount of emails due to those 2 users and IO disk usage is again close to 100%.

What causes the deferred queue to use so much disk activity, and how can I stop this for good?

Thank You