Re: Downtime warning
Note that it's easy to do the "group" thing at the receiving end -- it can involve either an SMS (single message store, where only one copy of the message exists, but each user gets a link to it -- that's how MS Exchange works), or simple copying of a document/file/record at the level of a server or two (default config for most other mail systems). Either way, it's done very quickly based on entries in a local directory. From the sender's end, if the email goes outside of the system, then each and every mail sent needs to be separately routed to each of the recipients. So you're not sending one email with 10,000 addresses on it, you're sending 10,000 emails, each to a single address (although each may carry a list of all of the recipient with it inside the routed message package so you can do silly things like "reply to all").
“Beware of bugs in the above code; I have only proved it correct, not tried it.” --Donald Knuth
"It was as if its architects were given a perfectly good hammer and gleefully replied, 'neat! With this hammer, we can build a tool that can pound in nails.'" -- Alex Papadimoulis (on TheDailyWTF.com)