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When I am sending a mail from my one mail account (Yahoo) to other (Gmail), Gmail automatically puts my mails into the Spam folder. What is the basic criteria to send an email by which mail servers cannot treat my mail as a spam?

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While the list of criteria used by GMail's spam filter is a trade secret, most spam filters are quite similar.

The list of spam criteria used by a very popular Spam Assassin spam filter is made public, together with weights.

Some of the most important ones, cherry-picked by me:

  • Character set indicates a foreign language
  • Body contains a ROT13-encoded email address
  • Message body has many words used only once
  • Message body mentions many internet domains
  • Received via a relay in Spamhaus XBL
  • Received via a relay in list.dsbl.org
  • Subject contains a gappy version of 'valium'
  • Subject contains a gappy version of 'viagra'
  • Attempts to disguise the word 'viagra'
  • Relay HELO'd using suspicious hostname (IP addr 1)

And so on.

There are some services that allow you to test your message for compliance with the spam filters and see what exactly is wrong with it — http://spamcheck.sitesell.com/ is one of them.

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    +1 I liked the answer, but won't spammers test their proposed spam with the likes of spamcheck.sitesell.com until it complies?
    – amelvin
    Commented Aug 17, 2010 at 12:50
  • They are already — spam-detection systems are Bayesian and learn whichever of their hundreds of parameters are most important. For example, exact match of “viagra” in the body text is not so important anymore, because spammers use it rarely, because it was very important before… …shortly, it's an equilibrium system similar to the classic hunter/victim model.
    – apostlion
    Commented Aug 17, 2010 at 13:05
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So Gmail has a learning system that is part based on all users and part based on a single user.

  • If Many users mark your mail as spam then it may get catagories as spam for everyone.
  • If that one user marks it as spam then it is only spam for them
  • If one user digs something marked as spam out of their spam folder then it is only not-spam for them

This is on top of the standard spam checks @apostlion described.

Easy answer is not to send spam, or things that users may consider spam.

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