All About Email Standards
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Addressing conventions must be uniform or easily parsed by different systems, otherwise there is no way to interoperate. The ISO X.400 addressing standard is one attempt at a universal standard for electronic message addressing, and one that still has significance for the Internet. However, the familiar email@example.com format was not always the only way to express email addresses.
The standard, globally unique Internet email addressing evolved over time, embracing and eventually replacing competing and alternative addressing schemas. Interoperable address representations are not enough for global messaging; directory services are also an important part of any discussion of Internet email standards.
There are rules for formatting an email message, for creating and interpreting message headers, and for using email addresses. Once the message is correctly formatted, enclosed within its headers, and given an appropriate destination, it must still be sent from its originator and forwarded on to that destination.
This is where things get more complicated, as a protocol defining how different systems are to deal with the task of getting a message from one place to another represents a higher level of complexity than merely defining what the message should look like. The transit of a message across the Internet from its source to its destination can be viewed as a single journey with three legs.
First, the message must get from the UA to an MTA. From there, it must travel from one MTA to another until it arrives at an MTA that can deliver the message to its destination UA. Finally, the message actually arrives at a destination UA from the last MTA. Looked at in this way, it’s possible to segment the journey into more manage-