Not necessarily. All it tells you is the router address that passed the email out to the public internet.
The public IP address could be NATed to multiple PCs on the private side of the router / firewall.
For example, I have 3 PCs, a server, a laptop, a Wii and 2 DS devices on my home network - all sharing a single public IP address for internet access via the firewall / router that's connected to the cable modem.
For a corporate network, the number of devices on the private side of the firewall could be very large indeed.
Further, if the public IP address is obtained via DHCP - as most home connections are - then it may change periodically. So you might also need to know when the IP address was assigned to the physical MAC address of the cable / ADSL modem - and have access to the logs of the ISP that assigned the address.
SD