EmailRelatedToId
Email Parameter
Active 
Release 7
Release 8
EmailRelatedToId
Parameter Value: [ID of an Account, Opportunity or other Salesforce object]
&EmailRelatedToId={!SalesforceObject.Id}
Email Related To
Sets the value of the Related To field.
The Related To field causes the sent email to appear in the Activity History related list of the object identified.
EmailRelatedToId cannot be related back to a who ID (Lead or Contact ID)