Answer: Application layer
Step-by-step explanation:
In both the OSI model and the DoD model we have the Application layer. The application layer in both the model is at the top layer. SMTP which stands for simple mail transfer protocol is one of the networking protocols used for sending and receiving mails across clients and receiver.
As the application layer is known to support application and protocols which makes it work as an abstraction layer for the network and the host.
So the SMTP being one of the networking protocols works at the application layer of the two most basic and important models.