SpamAssassin Rule: MPART_ALT_DIFF_COUNT
Standard description: HTML and text parts are different
The mail contains the content in both HTML and plain text format, but their content is (very probably) different - the number of words is significantly different in the two versions. This suggests that the sender is not using a normal mail client, and is attempting to evade filtering by using a message which looks different to humans and mail filters.
The default scores for this rule can be found in the online list of tests.