Workflow not triggered by Incoming emails?

Every time when a workflow is not started automatically, the first thing we need to check is about the "system account".   Normally when a user uploads a file to a document library (or insert an item into a list), the workflow will be triggered by that user.   So we just need to make sure don't log on SharePoint site as the service account.

However, when an email comes into the document library, it seems is uploaded by the domain user which is using the sender's email address, so why in some environment the workflow is not started automatically?   I am not sure about the exact reason, below is my guess.

Workflow is actually running under the process "SharePoint Timer". So the service account of "SharePoint Timer" is the real "system account". And incoming email file is actually uploaded by the web application service account.  So we need to make sure these two accounts are different.

(If you don't agree, please let me know.)

Below are the relevant screen shots.

Update (16/05/2011): The problem may caused by a SharePoint "bug".  In terms of this post, we need to run the command below to fix it.

stsadm -o setproperty -pn declarativeworkflowautostartonemailenabled -pv true