- /* we know that there are 4 characters after by - either the 4th one
- * is '\0' (broken header) or it is the first letter of the hostname
- * that last received this email - which we'll use to guess the right
- * from email address
+ /* sadly, the format of Received: headers is a bit inconsistent,
+ * depending on the MTA used. So we try to extract just the MTA
+ * here by removing leading whitespace and assuming that the MTA
+ * name ends at the next whitespace
+ * we test for *(by+4) to be non-'\0' to make sure there's something
+ * there at all - and then assume that the first whitespace delimited
+ * token that follows is the last receiving server