SpamAssassin Rule: MPART_ALT_DIFF
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. 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.