Salesforce marketing cloud emails have a ID and Name. But is there any external key associated with every email object? I can see something called a Customer key on emails created via content builder, whereas can't find the same in the classic content emails. Can thus customer key be considered as email external key? Our sales cloud counter part is trying to setup a campaign and attach the email send results to that campaign exact target configuration. They want the email name, ID and external key from marketing cloud. This is not a triggered mail.
Email object does have a CustomerKey attribute, as far as I know it is not visible on Email Studio page.
The only way for me to find the CustomerKey for a classic email is to make SOAP API call, RetrieveRequest for Email object, make sure "CustomerKey" in the return properties list.
This is the guide for Retrieve email via SOAP API