While MTU (Maximum Transmission Unit) mostly refers to computers, its value can be found on any communications protocol. For each type of communications media, there are quite a few MTU values that themselves vary depending on what the communications line is trying to send and how big that package is.
MTU defines a maximum allowable package size for specific communications transfers. The simplest way to understand the function is to relate it to post office standards. If you walk into the post office and find that they only have one size box you can use to ship items across the country, and that the items can only weigh a certain amount of weight, then you are limited to what size package can travel along a certain distance to a specific destination. It is the same with MTU definitions.
The MTU for any communications protocol varies. These variables depend on the speed of the cable or wireless communications device and the value set by the communications service. For example, you can hook your computer up to the Internet using a cable that allows, let's say, 10 gigabytes to pass through per second. However, if you have an Internet service that has configured its server to only accept and release 3 gigabytes per second, then your 10 gigabyte cable won't matter.
When trying to understand the effects of MTU, you must understand the makeup of the system it is defined under. Take the Internet, for example. You have to consider the MTU of the modem you use, the cable or phone line it is hooked up to, the server speed of the company who supplies the Internet and more. The effects of a defined vaue on any communications protocol is clear. No matter how fast one part of the system makeup is, a package is only as fast and large as the smallest Maximum Transmission Unit allowed on any one part of the system.
At this time in history, the largest MTU has reached about 9 gigabytes, which is the equivalent of about 9000 bytes. Of course, most maximum transmission unit definitions reach about half of that, if not less. It has the potential to reach higher as network technology improves to allow larger size packets to move through and higher speeds are reached. Again, these larger packets and higher speeds have to be available on every end of the network system in order to take true effect for the end user.
When you send a file through the email you will notice there is a limit of how large the file size can be. This file size is an MTU defining the largest package available on your network to send. Maximum transmission units are significant, for without them we would never know just how much we are allowed to send and could wind up spending hours trying to send a package that is just too big for the overall network to handle.