We send email using appengine's python send_mail api. 
Is there any way to tell why an email that is sent to only one recipient would be marked as SPAM.  This seems to only happen when appengine's python send_mail api sends to Gmail.
In our case we are sending email as one of the administrators of our appengine application.
And the email is a confirmation letter for an order that the user just purchased, so it is definitely NOT SPAM.
Can anyone help with this?  
It seems odd because it is only GMail users that seem to be reporting this issue and we are sending from appengine  (All Google servers)  I love Google but sometimes Google is stricter to itself than to others :)
I've added the spf TXT record to DNS such as "v=spf1 include:_spf.google.com ~all"
(I'm hoping that will help)
I've tried to add a List-Unsubscribe header to the email but it seems app engine python send mail does not support this header.
Thanks,
Ralph
My guess would be that the content of the mail looks "spammy" for Google, but you can do some things that might help you. 
I would suggest you, since this is a confirmation mail, add another admin for your app an email like: do-not-reply@domain.com and use that one for the confirmation emails. Add more text to the body and include the unsubscribe links as well, so your users will have the possibility to not receive more email from your app. Maybe you wouldn't like the last part, but you have to give that options to your users, so this email won't be marked as SPAM.
AppEngine python send email api is marked as SPAM by Gmail email reader