However, TCP is actually using IP so the software takes the TCP packets (which encapsulate the SMTP packets which encapsulate the original email) and encapsulates them in IP format which it then supposedly delivers. However, most computers cannot use IP directly. They use instead a variety of data link protocols, Ethernet being probably the most popular in a local area network. Ethernet has its own packet format, so it takes the IP packet (which encapsulates the TCP packet, which encapsulates the SMTP packet, which encapsulates the original email) and encapsulates that in its own packet format and sends that on the wire.

The lowest level, the "bare wire," so to speak, is called the physical layer, and it is governed by rules as much as any other, although they are hardware and electrical rules.