OK, this is going to sound crazy...
I was also experiencing an issue sending emails from Outlook 2016 on my PC and on my iPhone 8 Plus since the security update. I had followed the instructions in the Cox email to a "T," but could not send emails. I could receive them.
I resolved my issues on both by changing my password. I ran into this issue once a couple years ago, and resolved it by trying a different password. In this most recent situation, my password ended with a number. I changed my password, and everything began working immediately.
Like I said, it sounds crazy, but it worked for me.