Comment by Kovah

I think the biggest issue is IP reputation. There are good products out there to do all the heavy lifting, which prevent most of the issue with good default configuration. But no matter how good your setup is, if your IP is/was known for sending spam once, you're on some list with the big providers. If you don't already have an IP for years with a solid reputation, you're going to go through a lot of "please check your spam" communication. I know cases where even neglecting measures such as using DKIM is okay if your IP is okay. The other way around: you need to implement basically every measure possible to even get started being recognized as a trustable sender.