Well, it's the first barrier. If you mess-up really bad you will have bad deliverability across the board. The mistakes can be numerous: * bad setup like bro

Well, it's the first barrier. If you mess-up really bad you will have bad deliverability across the board. The mistakes can be numerous: * bad setup like broken DKIM * bad sending parameters like opening too many connections by IP to a given provider * bad content format like too many large images * bad actual content * bad recipient list management with lists containing lots of typos, none existent mailboxes, spam traps, etc * miss-managing unsubscribes * bad bounce handling If this happens, you can have some if not all of your IPs blacklisted, maybe even your domain. In worst cases, a whole IPv4 range is blacklisted. Then a second aspect to take into account is the open rate and the click through rate (people actually clicking a link in a marketing email), this one can vary depending on the company activities, and also the quality of their marketing content, but is generally in the range of a few percents at most. And then, you also have to take into account the actual impact in term of sells which will be even lower. Deliverability is import, it's not rocket science, but it's a lot of small items various people at various stages (tech, creatives, marketers, etc) must be cautious about which makes it actually quite hard.