I used to have same reaction regarding greylisting, but changed my mind around 5 years ago. Have yet to see any issue with it, and I assume this is because homebrew email servers are no longer common or even uncommonly in use. I guess it could happen, but I have yet to find this at work for those I host email for, and same goes for all my private emails. It can be that because many commercial email providers uses greylisting, users of homebrew mail servers has either fixed it or gone over to postfix/exim/sendmail by now.