Why does gmail think mail my server sends is a spam?

Are you following their bulk senders guidelines? (Like the 'Precedence: bulk' header...)

I don't think the HTML in your mail is valid. No root element, etc. If you send HTML-only mails, you should make sure its HTML is correct.

Or even better: also include a plain-text version of your message (people like me, who use console e-mail clients, will thank you!). IMHO including a plain-text version will also decrease the probability of your message being considered a spam (most spams I see are HTML-only, and this is a criterion for SpamAssassin at least).

Is it on a shared hosting or dedicated server. If it is shared if someone from the same IP has sent bulk emails(say 25 or more), then possibility of IP getting black listed is possible. Other reason for being spam is by using some of the spam words. Often the spam is done through algorithms using data mining techniques, which means you need to train algorithm several times emails to your email ID from different IPs and mark it is not spam which is not feasible solution. The other way of getting this out, is making your site atleast little popular by providing backlinks, usually the techniques used for SEO. I'm telling you this with my practical experience with many of my websites, which I ran for few times and shutdown.

If your server does not pass reverse domain lookup, gmail may put that inside spam.

Since your headers appear to be correct, it's probably keying on something within the message. One of the things that SpamAssassin (no idea if this is what Gmail uses) keys on is a very short message like the above containing a hyperlink or graphic, so you may benefit by actually making your message a little longer. One of the ways to find out is to send it to your own gmail account and when it appears in your spam folder, examine the headers there for any added spam information. It may contain clues as to what spam engine Gmail is using or what rules your message is breaking.