I first heard about it in about 1994 when a Unix guy I knew told me about a type of Unix that could run on regular computers. He loaned me a POSIX book, but I didn’t really hear anything until 98. I started getting fed up with all the problems with Windows 98, and I started installing it and breaking it on any machine I could get access too. I don’t know how many floppies I formatted with each disk image of RedHat and Debian. I broke the school network a few times with things like accidentally setting up a DHCP server. I sent a patch to the kernel. I Learned a whole lot those first years.

Check RBLs a lot of times services just use one of those, and they can be flaky. Usually, you can fill out a form and get reinstated.

https://mxtoolbox.com/blacklists.aspx