Finally ... our emails are being seen by Yahoo! not as junk anymore. Hurray!
However I notice that the Yahoo! receiving MTA adds in a X-YMailISG header. It's very large ... 2**10 bits?
Now that I've invested too large a chunk of my waking life in crafting our email headers I'm curious to know what an X-YMailISG header is. Can anybody tell me? Does it pose any security / authenticity issues? There's very little intelligible from Google results.
After many days tweaking TXT records in our domain's DNS zone file for SPF and DKIM, I have at last succeeded in generating email from our Drupal site that Yahoo! no longer marks as
X-YahooFilteredBulk and the excellent service firstname.lastname@example.org returns results that show the emails passing SPF, DKIM and Sender-ID checks and appearing to SpamAssassin as ham. Yahoo! even adds a Received-SPF: pass header.