It looks like you email is from an automated sender which is sending using one of Google's server. It may be connecting to GMail to send the message. Your server appears to have an SPF record, which neither includes the Google SPF list nor indicates that it should be enforced. Including a Sender header for email@example.com may help.
There are a number of factors that will make you email look like Spam before you even start sending it.
- Do you have a static address with a PTR record returning your hostname, or name on another A record for the address? (Does reverse DNS validation work?)
- Does your mail server announce itself using the same name the PTR record returns?
- Does you mail server have a SPF record permitting itself to send email?
- Is your address listed in list.openwl.org?
Failing the first two tests are strongly indicative of Spam, or an automated mail source. Person to person e-mail almost always pass these tests, while Spam often fails. The next two tests are indicative of non-Spam.
Once you have started sending the email there are other tests that apply.
- Both the return path address (envelope address) and From address should have an MX record pointing to a working mail server.
- The return path address should match the From or Sender address header.
- Both domains (if different) should have a working postmaster address.
- Both domains (if different) should have SPF records permitting mail from the sending server.
- Previous history of both email addresses and their domains may be considered (autowhitelisting / autoblacklisting).
- Does the message have a valid DKIM signature.
- Any other rules that the email administrator deems appropriate.
I find I get a relatively high failure rate on DKIM signed documents from automated senders. This is mainly related to the public keys not being available from DNS.