Google? Ik kom met search term "net send" en "linux" en "samba" uit op bv.
http://www.winnetmag.com/Articles/Index.cfm?ArticleID=8897, welke jouw vraag behandelt.

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Sending Messages to Workstations
You might know the Net Send command, which lets you send realtime, pop-up messages to machines on a Microsoft network. The command
net send jane hi there!
causes a box containing the words "hi there!" to pop up on Jane's machine. You can direct the Net Send command to machine names as well as to usernames, so if Jane's machine is named \\PC004, you could as easily type
net send pc004 hi there!
Smbclient performs this task with the -M option. The command smbclient -M pc004 establishes contact with \\PC004 and waits for you to type your message. When you end the message (by pressing Ctrl+D), smbclient sends it. As with many UNIX and Linux commands, the option's case is significant—the -M option must be uppercase.