The following mental ray shader parameter types are recognized as message attribute by Maya: light, geometry, color texture, scalar texture, vector texture, lightprofile, shader.

Though it is possible to connect any node to these attributes, you should make only those connections that Maya expects, otherwise a crash or rendering abort may occur. That is, if you drag a node to the message attribute in the Attribute editor, the message attribute of the source node will be connected to the attribute (for example node1.message > mrShaderNode.shader), but only the following connections are recognized and will work in Maya:

• light : light transform node. For example, pointLight1.message > dgs_material1.lights[0]
• geometry : geometry shader. For example, mib_geo_sphere1.message > mib_geo_instance1.object
• shader : mental ray shader. For example, mib_illum_phong1.message > mib_geo_instance.material
• color texture, scalar texture, vector texture: mentalrayTexture node. For example, mentalrayTexture1.message > mib_texture_lookup1.tex. The button next to a color/scalar/vector texture attribute in the shader's attribute editor will create a new mentalrayTexture node, and connect it to the attribute.
• lightprofile : mentalrayLightProfile node. For example, mentalrayLightProfile1.message -> mib_light_photometric.profile. The button next to a lightprofile attribute in the shader's attribute editor will create a new mentalrayLightProfile node, and connect it to the attribute.
• input-to-input connections, that let you render more dependency graph connection in Maya scenes.
  NoteBreaking an input-to-input connection causes all the dependent nodes in the chain to lose their inputs.