Email reliability in this Internet-based world.
It ought to be reliable, shouldn’t it? But, it is not.
You can thank all the spammers for much of today’s unreliability, as email spam filters at the Internet Service Providers and on our on machines manage to misclassify emails — both false negatives (where they let spam through to annoy us) and false positives (where our good emails get trapped and discarded).
Read more in my article Email Reliability in this Internet World