Sign up for a Tuffmail account, as well as accounts from similar services. Make sure that the SPAM filtering is set to the MAX for each account. Make sure that the SPAM filtering message headers are turned on. Then, do all your testing my sending to these accounts. Observe the headers set by SpamAssassin and other spam filters. They will tell you exactly what is going wrong.
Send the absolute bare minimum of email that will get the job done.
In particular, don't use email to send invites. I, and other people I know, file invites and similar requests as SPAM specifically to cause problems for sites like yours. I promise you that if I get any email from your website, it will be forwarded to SpamCop and other places, whether or not my friends requested you to send it to me.
Check your IP addresses of your servers to make sure they are not on any blacklists. Similarly, check the blacklists to make sure your domain name itself isn't on them.
Make sure that your mail server has a domain name, a static IP address, and make sure that the reverse DNS resolves back to its domain name. This is needed to get past ESMTP ELHO filters.
Make sure that you have a valid and restrictive SenderID/SPF record in your domain's DNS.
Make sure that the syntax of all your SMTP commands is compliant with the SMTP specification. Make sure the syntax of your messages are compliant with RFC2822, MIME, and related standards.
Ensure that you are using the following in the SMTP handshake (square brackets used instead of Less-Than and Greater-Than to get through the filters on this site):
EHLO onista.com
MAIL FROM:[XXXX@onista.com]
Make sure that the From: header is an address from your domain: From: XXXX [XXXX@onista.com]
If you refuse to do that (because you want to forge your user's email address in the From: header):
From: Joe Blow
Sender: XXXX [XXXX@onista.com]
Avoid HTML MIME messages. If you refuse to avoid HTML messages, avoid messages with images. If you refuse to avoid messages with images, then embed the images in the MIME message instead of hotlinking them to your site.
Implement the recently-finalized DKIM standard.
Ensure that your WHOIS information for your domain is accurate, up to date, and has contact email addresses with your domain name in them.
Pay money to GoodMail and related services. You WILL have to do this eventually, because you WILL be sending a lot of email that people simply don't want to receive.
I did come across this article that may give you some help: http://help.yahoo.com/l/us/yahoo/mail/original/abuse/abuse-5...
AOL has a pretty good system setup for monitoring and reporting why your emails are getting marked as spam. If I remember right Hotmail has a program where you can pay for priority delivery. And I could never figure out a way to ensure delivery on GMail.
Yes, Gmail is really a challenge. I am trying to play around our email headers to see if it helps, but so far no luck.
Will test on hotmail and AOL soon.
Thanks again.
We are working on developing Social Marketplace called Onista (http://www.onista.com) and we are in QA cycle right now on our test server. Most of the emails that our application is generating for users (like Invite emails or registration emails) are being delivered to bulk/spam folders on yahoo and gmail. Any idea on how can we fix the issue?
Thanks in advance.