I am going to throw this out there, after reading this post I noticed that I was no longer receiving email anymore for someone replying to a post etc. When sending a test email from the ACP I still was not able to receive it. I went to my server and looked at my logs and it had something to do with DMARC. Since I use Cloudflare I had to go in there and configure a few things, but now everything is working fine again. So, I don't know if you use Cloudflare but if you do check your settings, they must have made changes here a few weeks ago, because it always worked before then. I don't know if it's related to your scenario or not.