Most of what goes wrong with an email send goes wrong quietly. Here are the three mistakes that cost the most, and what they have in common.
An email that doesn't arrive says nothing. There's no error dialog, nobody calls, and the sender only finds out if someone happens to ask why they haven't heard anything.
The most expensive mistake we've made was letting a misspelled address through without telling anyone. That person could neither receive the sends nor sign in to correct the address, because sign-in goes to the same place.
What all three mistakes had in common is that the system knew about the problem and told nobody. That's the category of fault we look hardest for now.
