Need help making my validation email less offensive to spam filters

[DWilliams]

My application has an email validation feature like so many others, but on my first test deployment many of my users are reporting that it's getting sent to their spam folders.

 

Here is the basic contents of the email:

 

You are receiving this email because you requested an key to validate your email account with the <site name> installation of SMSWebAlerts
========================================================
If you received this email in error, please delete it. If not, your activation key is 117390f8956f242dca9d3d9c8eaaf7c5. You can enter it manually on the validation page or visit this URL to activate your account:

http://www.example.com/validation.php?key=117390f8956f242dca9d3d9c8eaaf7c5

 

What part of that is triggering the spam filters and how can I reword it to actually hit the inbox?

[Yucky]

Ask them to find out why it triggered the spam filter. He might be able to find out by checking the headers.

[PFMaBiSmAd]

Does your sending mail server have an SPF DNS record and what mail header fields are you using when you send the email?

[DWilliams]

Does your sending mail server have an SPF DNS record and what mail header fields are you using when you send the email?

 

I'm not too sure on the techincal details of it honestly. I'm using the PHPMailer library using its default settings. I'm sending via GMail's free SMTP service currently.

[PFMaBiSmAd]

So, you are sending through a gmail server and the receiving mail server(s) is(are) voting the email as spam?

 

What are you setting the From: address header to? An email address hosted at gmail, since that is where the email is being sent from?

[DWilliams]

So, you are sending through a gmail server and the receiving mail server(s) is(are) voting the email as spam?

 

What are you setting the From: address header to? An email address hosted at gmail, since that is where the email is being sent from?

 

Yes, I set up a GMail account and am using its SMTP login details to send email. GMail overwrites whatever From: header you send with the actual email address it was from so that shouldn't be an issue.