Fix
We've been seeing more and more outbound delivery delays recently. Most users have not witnessed this, as it represents a very tiny fraction of email our users send. We've made several attempts to mitigate this, which has even further reduced the scope of these events. However, the problem being statistically uncommon hasn't done much to provide comfort on our side, considering that outbound email is kind of our point of pride.
The cause of this has finally been tracked down to an old deployment (stopforumspam recipient block list) that was "fine" at the time, but given the growth of our platform it slowly became a larger issue. A change in how we deploy this is now reducing outbound email delays, to the point that it should completely do away with them.
If you experience outbound delivery delays, we do encourage you to open a support ticket. The chance that any delays you saw were related to this update are roughly 1 in 300,000. So if your emails are being delayed somewhere, let us check and tell you where (if we can).