To approve worklist item just by sending a mail

If you are open for a slightly different approach, you could easily add links directly in the email, one that says Approve, the other Decline (or whatever actions you have). The URL of these links will point to a generic page as well as 2 Query Strings parameters:

1. 
   
2. SN: Standard serial number that is contained within all Worklist strings
   
3. Approval: For the Approved link, its approved, declined for Declined link, etc.

The generic page will then use the SN to open the Worklist item and the Approval as the Action to execute. Once executed, the web page will just display a Success or Thank You message.

This is a very easy approach to do approvals directly from email, the downside however is the user needs to be online to do so. They will also see an additional web page (you can decide whether this is good or bad).

If you require offline approval through mail, you will need to some email parsing on the initial sender's mailbox (typically the service account). A bit more tricky though as this account will need to have impersonation permissions and there's always the possibility of users making typo's, causing parsing to fail.

You can also look into using Exchange forms. Possible, but again, quite a lot of work with (usually) not that much return.