The domain after the @-sign is known on the Internet, and serves as the gateway to the unknown destination host, which may be on a private LAN or other non-Internet network. The address before the @-sign must be a valid address on the target system, except that that "@" is replaced by the "%".
For example, anders@hq.4thbn is a valid address on a private mail
sysem on an Army base. If you were on base, that is all the e-mail address
you would need. But from off-base, you must specify the Internet gateway
that recognizes the local mail organization, and append that gateway name
to the local address. Since you don't want TWO @-signs in the address,
you change the @ in the local address to a %, then add the "@gateway" name.
The resulting address is:
anders%hq.4thbn@benning-emh2.army.mil
Similarly, BITNET addresses were directly deliverable only on the (now defunct) BITNET. From Internet sites, a BITNET address such as 1234p@navpgs was deliverable when 'hacked' to use an Internet/BITNET gateway, where transfer of the mail and protocol conversion took place. To be valid from Internet, you could use 1234p%navpgs.bitnet@cunyvm.cuny.edu. The City University of New York was known to Internet, and was a transfer point for BITNET mail known as an InterBIT Gateway.