Although it is a good practice to manage the RFC 2822 header fields, or X-Headers in your email system, SharePoint does use some X-Headers for alert and workflow emails that need to be allowed in messages for all functionality to work. For instance, the edit or open task button in Outlook client (see below in red) may not work if the X-Header to connect to the task list is not present.
When clicking on the open or edit buttons, you may receive an error like the following:
An error occurred while retrieving the workflow task details. This could be caused by not having connectivity to the server or because the task no longer exists.
Here is a list of some of the X-Headers used by SharePoint in various versions:
X-Sharing-WSSBaseUrl, X-AlertWebSoap, X-Sharing-ItemID, X-Sharing-Remote-Uid, X-Sharing-Config-Url, X-AlertServerType, X-AlertWebUrl, X-AlertId, X-AlertTitle, X-Sharing-Title
So if your email servers are using any kind of message filtering, like the Exchange Server 2010 Header Firewall and wanting to have full functionality in Outlook for SharePoint emails; make sure that the above X-Headers are not blocked.