What does X-Spam-Status mean ?

SpamAssassin adds an extra email header such as

X-Spam-Status: Yes, score=21.6 required=4.0 tests=BATMAIL,BAYES_99,
DATE_IN_FUTURE_06_12,FS_REPLICA,FS_REPLICAWATCH,RAZOR2_CF_RANGE_51_100,
RAZOR2_CF_RANGE_E8_51_100,RAZOR2_CHECK,RCVD_IN_PBL,RCVD_IN_SORBS_DUL,
RDNS_DYNAMIC,URIBL_AB_SURBL,URIBL_BLACK,URIBL_JP_SURBL,URIBL_WS_SURBL
autolearn=spam version=3.2.1

These are visible when the a mail client is configured to "show full headers", or a similar option.

The fields are the following:

• Whether the message is spam (yes/no)
• The total score for the message (can be negative if whitelisted)
• The score that would be required to be classed as spam
• The comma-separated list of tests that returned non-zero value
• Whether autolearn learned the message as spam or ham
• The version of SpamAssassin that was used
  

To find out why a message was classified as spam (got the score it did), check the tests listed.

A list of the rules, short descriptions and assigned scores is at tests. For many rules, a Wiki entry gives more details.

If a message is classed as spam, by default it is attached to a text message that gives the tests failed, points assigned, and short descriptions.