Why Ending Up in Gmail Spam Folder

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 [452533]

You may also go to: https://www.mydomain.org/index.php?a=dirLog&e=first.last@gmail.com&i=10408259
Paul KonstanskiProject SpecialistAsked:
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Dave BaldwinFixer of ProblemsCommented:
Because it looks like spam that is 'phishing' for people's passwords.  I don't know what you can do to change that because you are asking them to login.  Maybe you can try different ways of saying it to see something works better.
Ray PaseurCommented:
You can ask your clients to whitelist your domain and sending email address, and you can use their name in the message.
Mark BradyPrincipal Data EngineerCommented:
I know this is a pain in the butt, but, there are no buts about it. You need to tell all your recipients to add the email address you are using to send this email to their address book and it should no longer go into their spam/junk folders.

Trying to have your email address taken off a blacklist is a long and frustrating process to say the least!
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Paul KonstanskiProject SpecialistAuthor Commented:
Since posting this and seeing some of the first comments, I did a little testing.  The one thing I didn't mention that I guess now is quite relevant is that the "sender" in this case was "webmaster@mydomain.org"

Once I changed the "sender" to more of a person type of email "wb.mster@mydomain.org" they stopped going to spam.  So far that seems to be working.
dhsindyRetired considering supplemental income.Commented:
^^ I tested it by sent from a non-gmail account to a gmail account.  It went to the Inbox - no problem.

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Mark BradyPrincipal Data EngineerCommented:
Paul KonstanskiProject SpecialistAuthor Commented:
After some time went by, it continued to end up in people's email spam boxes, even when I tried changing some information. The bottom line is that it's a tricky business.  I'm still experimenting.
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