fast notification of users during emergency?

[vbfox]

What is the fastest and cleanest way to deliver a notice to all users?

Thinking in terms of campus emergency.

The Cyrus MOTD function is a one-liner, and only works for IMAP clients.

My first thought was to create a message 0. and then hard-link it into all
user inboxes and reconstruct. I checked the code though and it seems only
messages 1. and higher are recognized.  Scratch that idea as we would run
into collision avoidance problems using regular numbers.

We have 2 Cyrus mail-stores about 30K users each.  I assume there is some
practical limit to recipients, or can I just list all users and send a 
single
message to the entire list at one shot?