For example, a user at Imperial College (IC) would have been: user@uk.ac.ic, rather than user@ic.ac.uk as they might today.
Email gateways existed to convert between the different systems, but when all country codes were added to the internet, some addresses were simply ambiguous. (An address ending in .cs might be either czechoslavakia or a computer science department.)
The main email software responsible for this slicing and dicing was known as PP, which helps explain the excellent quote on ASR (alt.sysadmin.recovery) a few years later:
http://home.xnet.com/~raven/Sysadmin/ASR.Quotes.html [search page for "mail transfer agents"]