Changing how recepient value is getting handled.

This was trying some handwaving recepient checking (hey maybe it's
needed).  Let's try this instead.

docs/courier-to-mailman3.md

@@ -36,3 +36,5 @@ This is our current version trying to communicate between courier and mailman3.
   * Let's try that instead?
 * Oh man...  Variable not properly defined (or more derferenced.)
 * OK, it's failing further along, so maybe this works?
+* I see...  
+  * The parsing of the environment variables is failing, but let's try this.

source/courier-to-mailman3.py

@@ -54,7 +54,7 @@ try:
         arg_ext = arg_ext_tmp if arg_ext_tmp != "1" else ""
         lmtp.sendmail(
             os.environ['SENDER'],
-            os.environ['EXT' + arg_ext] + "@" + os.environ['HOST'],
+            os.environ['RECIPIENT'],
             #sys.stdin.buffer.read()
             content
         )