For password recovery purposes, I send out an email. Although it has worked fine for a while, most recently gmail is now putting this email into people's "Spam" folders and it gives the reason:
Why is this message in Spam? It's similar to messages that were detected by our spam filters
I've tested a few other things (like other emails with different content and they come through fine), so I'm looking for hints as to why the following subject and content may be causing a problem. Any successful help will be rewarded.
The email is being sent using the qmail program and a php script.
The following is the subject line and content of the email.
Validation Code (452533) Request
A request has been made to have login information sent to this email address. If you did not make this request, you can ignore this email because the requesting individual can only gain access with the code provided below.
The six-digit validation code is used to return to an existing registration by entering this code when asked on the web site. You may also click the link below to return to your registration. Upon successful validation, you will be granted access to the site.
This code will be valid for six hours from the following time: Friday October 18, 2013 at 13:56 GMT
Validation code is 
You may also go to: https://www.mydomain.org/index.php?a=dirLogemail@example.com&i=10408259