The Delivery log should be the log that contains outbound SMTP deliveries. However, I could also agree that separating that out of the delivery log into an "SMTP Out" log could be useful too since we've been starting to put more and more info into the delivery log. While having lots of info in delivery isn't necessarily a bad thing it could potentially a bit harder to look through.
Sorry was a bit confused in my response you mentioned
"the smtp log file contains all incoming smtp traffic but also all outgoing smtp traffic." Which is incorrect but you corrected yourself later on. Unless I missed something. SMTP right now should only contain inbound SMTP traffic.