Three days after I sent an email to the subscriber list of one of my brands, a reader replied with a screenshot. The subject line was fine. The sender was fine. The body was a smear: every line of text stacked onto the same narrow band of pixels, sentence printed over sentence until the whole message was one unreadable stripe.
That screenshot was the first evidence of what the send had actually done inside real inboxes. The template bug itself had been caught and fixed the same day the email went out — I will come back to that, because it makes this story worse rather than better — but the fix faced forward, toward future sends, and nobody looked back at the copies already delivered. So the first report from the receiving end was not a bounce report, a monitor, or a dashboard alert. It was a human being, doing unpaid quality assurance for a list they had voluntarily joined, three days after the fact.
This is the postmortem, and the four guards that now exist so no reader ever holds that job again.
What actually broke
The email platform I use has its own template format, and that format expresses line height as a percentage of font size. I had given it the value 1.65 — a perfectly ordinary number if you have spent your life reading it as a CSS ratio, where it means "165% of the font size, comfortable reading rhythm."
