The issue on our primary mail server has finally been resolved, and all messages in the queue have been delivered. As expected, once we had access it only took a few seconds.
We will post a post-mortem in the next couple of days … hopefully. I can’t exaggerate the extent to which numerous unrelated events have piled on top of one another — even in the last few minutes! — to prevent an earlier resolution of this problem, and at this point I can’t predict whether or not more issues will prevent the posting of the post-mortem. However, I’m finally taking a breath, as this issue (amongst other things) is finally resolved.
I do once again extend my heartfelt apology for this incident, and I will do everything in my power to review the cascading failures — all not even related to the mail server itself! — that led to this not being resolved much, much sooner.