Help, Registration emails go to spam inbox

Question

I just started offering user accounts on my website (large PR6 site, with a good reputation) and I noticed that the registration emails almost always go to the receivers spam box.

So far we have only sent out a few emails, so its not like we've been slamming out thousands.

I send the emails via PHP with the mail() function. Below you can find the headers that i send with the mail() function.

$headers  = 'MIME-Version: 1.0' . "\r\n";
$headers  .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers  .= 'From: Domain.com <noreply@mydomain.com>' . "\r\n";

Is there a way, or a trick to get round these spam filters? Its clearly not spam what I am sending out. The registration email contains 3 links.