Address buses are made up of a collection of wires connecting the CPU with main memory that is used to identify particular locations (addresses) in main memory. The width of the address bus (that is, the number of wires) determines how many unique memory locations can be addressed.
It really would depend on what E-Mail system you're using. If say, you're using G-Mail, the Recipient's address would be placed simply in the "To" bar right above the "Subject" bar.
