Negate.

src/mailman.scm

@@ -163,7 +163,7 @@
    (let loop ((ml-emails (mailman-list-members ml)))
      (when (not (null? ml-emails))
        (let ((email (car ml-emails)))
-	 (when (member email emails)
+	 (when (not (member email emails))
 	   (mailman-ensure-not-member ml email))
 	 (loop (cdr ml-emails))))))