We are currently sending emails through Exact Target utilizing a data extension that passes subscriber ID and email address as attributes. Is it possible to pass this back to the SFDC Contact Object and store the email on the email send related list? Based on posts I've found, I am not sure the best approach. Am I off to think we need sync the contact object as a synchronized data extension and pass the sfdc id as an attribute? Does the 18 digit contact id become the subscriber id? I also read it is possible to upsert contact using an amp script. If I am completely off, I appreciate any help!
You have several options available to you. If you are using MC Connect, then yes, ideally your SubscriberKey or ContactKey should be equal to the Salesforce Record ID. This is how the clouds are intended to work together.
Based on your requirement, and if this is not the case, you have a couple of options.
You can use AMPScript to upsert a task (an activity), that just informs that they were sent an email, etc.
You can, in Journey Builder, create such a task and populate it with data
However, by far, the easiest would be to use Record Id as Subscriber/ContactKey, as this would give you things a lot easier:
A related list showing all the emails that has been sent to the subscriber, as long as you do it in the right ways (through Journey Builder, data extensions in the Salesforce DE folder or initiated via Salesforce)
You get to separate marketing emails from user email
You get to see the email in full + get tracking data as well
Let me know if there are more questions.