Overview:

Marked_Spam_vod is the value that the status of a Sent_Email_vod__c record is often updated to when the email is marked as spam by the recipient.

When an email recipient marks an email as being spam, usually their email service provider passes a response back to the sending domain indicating that the email is marked as spam, and that no future emails to this address should be sent or will be accepted. But there can be cases where a recipient marks an email as spam, but the Sent Email record is not updated to show this. 

Root Cause:

Status_vod__c not updating to Marked_Spam_vod can happen because some email service providers do not return a response that the email was marked as spam.  Some email service providers consider it a security risk to send a response to a possible spammer if the message is marked as spam.  This is considered a security risk because a Marked as Spam notification notifies the spammer that the email address is a valid email address.

Solution: