Hotmail and Gmail mark emails as spam [duplicate]
Solution 1:
Your message is being rejected as spam because it precisely matches the profile of a very common malware distribution scheme: messages claiming to be order or shipping confirmations, with PDF attachments.
My suggestions:
Clean up the grammar. "This is purchase order document for purchase order #1001"? Broken English sounds incredibly spammy, and although I claim no insider knowledge, you can assume that Gmail considers poor grammar as one factor in scoring potential spam messages. A real purchase order would just have a heading of "PURCHASE ORDER" followed by the actual purchase order content. It would not first say, "Hello friend, this is purchase order for buy definite article."
Replace the PDF attachment with inline content. Exploiting Adobe vulnerabilities via maliciously crafted PDF attachments is an extremely popular way to infect remote computers with malware. If I were Gmail, I would mark a PDF attachment from an unknown sender as spam, too.
Get rid of the "post-only mailing" mumbo-jumbo that almost certainly boosts your Bayesian spam score, and send the message from a valid reply address. If you are really sending genuine purchase orders in this way, you are going to want to know if they bounce, and you are going to want the vendor to be able to reply, right? Right.
You need to include your company's name and address, links to your web site, valid contact e-mail address and phone information, etc. The more anonymous and "hit-and-run" your message appears to be, the more likely it is to be classified as spam.
Unless there is a very good reason not to do so, you should end your SPF record with
-all. The entire point of an SPF record is to positively identify valid sender IPs and ban everyone else. You should not leave it up to the receiving mail server to decide whether an unlisted sender IP is valid.