A word which describes a representation of new technology by an older technology

As far as I know, it's called skeuomorphism. It is a broad term and has many definitions.

Skeuomorphism: A skeuomorph is physical ornament or design on an object made to resemble another material or technique. The phenomenon is called skeuomorphism. This definition is broader in scope, as it can be applied to design elements that still serve the same function as they did in the previous design.

On a smaller scale, the icons of GUIs may remain skeuomorphic representations of physical objects that are not longer directly applicable to the task they represent.

Example: a drawing of a floppy disk to represent "save".

[Wikipedia]

And here's the example you've given:

The envelope is the de-facto symbol for email and SMS messages. It offers a nice distinction between read and unread - they become opened and unopened envelopes. [Skeuomorphism - BBC]