It took about 30 years until most email users could correspond with most other email users. Until that time, you would often be limited to email with other users of your particular email system, or with other email users at work.
For transparent global connectivity, a number of things had to put in place, the principal of which were:
- A standard message format. The MIME format was ultimately adopted
- A standard email address format. The main battle was between X.400 addresses, and the much simpler internet mail addresses
- A standard method of exchanging messages. The main battle was between X.400 ADMDs (an electronic version of the international postal system), and direct internet-based exchanges using SMTP
- A widely-deployed, standardized, and inexpensive data network, as provided by the global internet
- Common support for the associated standards by most commercial email systems and products
We tentatively estimate that as of 1997, a user of one email system could reasonably expect to be able to exchange email with a user of a different and arbitrarily chosen email system.--David Ferris