Spam/Junk filters determine whether or not an eMail is classified as spam based primarily on content. What does the eMail look like? Does it contain a lot of markup, or is is just plain text? Does it mention "automated response" or "Viagra"?
Also, I would recommend sending the mail through your mail server (instead of just using mail() / sendmail) Your mail server should have a non-generic-looking RDNS entry which will help with big providers.
There are lots of other things to help ensure your mail is delivered, especially to companies that use whitelists, but the content of the message is the first thing to look at is the content. If you could post an example complete with headers it may help.
As for added security (beyond simply getting the mail through) I would suggest an RSA signature.
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