senza programmi di terze parti è possibile inviare un messaggio ad un mac in rete?
anche con terminale

TALK(1) BSD General Commands Manual TALK(1)

NAME
talk -- talk to another user

SYNOPSIS
talk person [ttyname]

DESCRIPTION
Talk is a visual communication program which copies lines from your ter-
minal to that of another user.

Options available:

person If you wish to talk to someone on your own machine, then person
is just the person's login name. If you wish to talk to a user
on another host, then person is of the form `user@host'.

ttyname If you wish to talk to a user who is logged in more than once,
the ttyname argument may be used to indicate the appropriate
terminal name, where ttyname is of the form `ttyXX'.

When first called, talk sends the message
Message from TalkDaemon@his_machine...
talk: connection requested by your_name@your_machine.
talk: respond with: talk your_name@your_machine

WRITE(1) BSD General Commands Manual WRITE(1)

NAME
write -- send a message to another user

SYNOPSIS
write user [ttyname]

DESCRIPTION
The write utility allows you to communicate with other users, by copying
lines from your terminal to theirs.

When you run the write command, the user you are writing to gets a mes-
sage of the form:

Message from yourname@yourhost on yourtty at hh:mm ...

Any further lines you enter will be copied to the specified user's termi-
nal. If the other user wants to reply, they must run write as well.

When you are done, type an end-of-file or interrupt character. The other
user will see the message `EOF' indicating that the conversation is over.

You can prevent people (other than the super-user) from writing to you
with the mesg(1) command.

If the user you want to write to is logged in on more than one terminal,
you can specify which terminal to write to by specifying the terminal
name as the second operand to the write command. Alternatively, you can
let write select one of the terminals - it will pick the one with the
shortest idle time. This is so that if the user is logged in at work and
also dialed up from home, the message will go to the right place.

Options available:

...person if you wish to talk to someone on your own machine, then person
is just the person's login name. If you wish to talk to a user
on another host, then person is of the form `user@host'.

...When first called, talk sends the message
Message from TalkDaemon@his_machine...
talk: connection requested by your_name@your_machine.
talk: respond with: talk your_name@your_machine

to the user you wish to talk to. At this point, the recipient of the mes-
sage should reply by typing

talk your_name@your_machine

The write utility allows you to communicate with other users, by copying
lines from your terminal to theirs.

When you run the write command, the user you are writing to gets a mes-
sage of the form:

Message from yourname@yourhost on yourtty at hh:mm ...