Does this work?

Maybe?

docs/courier-to-mailman3.md

@@ -39,4 +39,6 @@ This is our current version trying to communicate between courier and mailman3.
 * I see...  
   * The parsing of the environment variables is failing, but let's try this.
 * That's not doing it...  Might need to create users?
-* OK, let's see if what I need is in the headers...
+* OK, let's see if what I need is in the headers...
+* Using the "To" header...  Let's see if that gets us anywhere.
+* It's not finding it.

source/courier-to-mailman3.py

@@ -47,7 +47,7 @@ try:
         content = sys.stdin.buffer.read()
         #content = content.replace(b'\r\n', b'\n').replace(b'\r', b'\n').replace(b'\n', b'\r\n')
         header = content.strip()
-        headers = HeaderParser().parsestr(header.decode('utf-8'))
+        headers = dict(HeaderParser().parsestr(header.decode('utf-8')))
         header_to = headers[To]
 
         # See <https://qmail.notqmail.org/qmail-manual-html/man8/qmail-command.html>