In order to provide for the exchange of email on different computers, using different email programs, it was necessary to define standard formats for the transmission and exchange of email. The computer industry developed two main approaches:
- Internet-based standards
The core internet-based standards for email were defined between 1982 and 1988:
- The POP standards provide for retrieving messages from a message store, depositing them there, and deleting them from the message store.
- The IMAP standards provide for much greater control over access to the message store.
- The SMTP standards provide for transfer of email.
By about 1994, the internet-based approach became dominant. This was mainly due to the:
- Spread of the internet
- Simplicity of using internet rather than X.400 email addresses
- Cheapness of internet mail. With X.400, service providers would charge for email transmission; with internet mail, once you had an internet connection in place, it was free.
The MIME internet-based email standards were published in 1996. These mainly provide for file attachments and rich text in messages.
Additional internet-based standards have been defined, for example, for the transmission of encrypted messages; however the POP/IMAP/SMTP/MIME standards are by far the most important.