Interesting story, but even if you had 100% code coverage in your test, you probably would have forgotten to add a test for this problem. A good test coverage is useful, but does not guaranty to catch all the possible problems. It's nice to read that you added some checks to avoid repeating this error in the future.
Thanks for reading. Although I now realized that I reached a tipping point mid December when I had more users. I should have added some test in critical parts earlier.
The product is email centric so would have made sense to test this part.