In a previous job, my team was responsible (among other things) of the company's newsletter. Nothing great but nothing spammy either, and we made sure to follow

In a previous job, my team was responsible (among other things) of the company's newsletter. Nothing great but nothing spammy either, and we made sure to follow anti-spam law by having the newsletter subscription being opt-in and having a clear and easy unsubscribe flow. We didn't send the emails ourselves, but rather used a big player in that area. What I learned is that sending emails and not getting treated as spam by any of the big email providers is an absolute mess of tinkering and hoping it'll work. Not all of them would categorize our emails as spam, but Outlook.com started doing it out of the blue, and even a year later we didn't have a clue as to why or how to make it stop. Support for email senders is as good as for users, meaning getting anything else than a canned response wasn't happening.