Wood Notes

Used as a quickly disposable method of communication, wood notes have been employed in government and military applications for millennia, even before the demonic conquest. They are meant to be read and disposed of in the nearest fire in order to prevent others from finding them. However, they are often still written in code in case they are intercepted. 
The amount of magic needed to sent a wooden block versus paper is much higher. However, wood is used because it is more difficult to intercept via magical trapping due to the higher magic involved. Most trapping methods rely on capturing magical energy, but are only able to take in a certain amount. Paper is unable to hold as much as pure wood and thus is easier to catch.

Utility

Wood notes are used to quickly and securely communicate important information. Interception if difficult and thus the technique is often employed for military and government applications. Once the magic reaches its intended recipient, the wood reforms far easier and can be reconstituted into the wooden block it originally was.

Manufacturing

Different types of wood can be used, but some hold magic better than others and thus can be made into more secure missives. Small pieces of these woods are more expensive than lumber made from the same material as sellers know they will be used for discreet communication. To get around them, those wanting to use the wood will buy lumber and simply cut the pieces themselves. 
In order to have the wood absorb magic, the sender must be familiar with their recipient's magic as well or have something in their possession connected to that person. This allows the wood to attune to the recipient in order to find them more efficiently. Once the attunement is complete, the sender will infuse the wood with their own magic, however much they deem necessary, and then burn their note onto the wood.