NET.exe

The NET Command is used to send network messages as follows:

Network broadcast messages use "messaging names". A messaging name can be the ComputerName, your UserName or any other name you add as part of the NET NAME command line.

To send a broadcast message to another user
NET SEND j_smith Meeting changed to 3 p.m

To send a broadcast message to another machine
NET SEND mc_144 Meeting changed to 3 p.m

To send a message to all users who have a session with the server
NET SEND /USERS This server will shut down in 5 minutes.

To send a broadcast message to the whole domain
NET SEND * This server will shut down in 5 minutes.

To send a broadcast message to another domain
NET SEND /DOMAIN[:other domainname] This server will shut down in 5 minutes.

Messaging names:

To add an additional messaging name (for receiving messages.)
NET NAME extraname /ADD

To remove a messaging name (n.b. it is not possible to remove the ComputerName with this command)
NET NAME extraname /DELETE

To use NET SEND the Messenger service must be running, many organisations choose not to run this service.

Related:

MSG - Send a message
NET - Manage network resources
MAPISEND - Send email from the command line
Q330904 - Messenger Service Spam