A little button added to the SendMailFrame named "Send to Guild" that will send a mail with the currently entered subject and body to everybody in your guild.

This addon was initially meant for one thing and one thing only - to enable my guild leader to send out his newsletter.

I recognize that it would greatly benefit from some features like custom mailing list and such, but making this possible will most likely take the double amount of work or more than this addon has taken so far. I'm currently busy lvl'ing my alt to lvl 70 before WotLK and I'll probably be even busier after the 13th. An extension of this is definitely on my TODO list - but don't keep your hopes up for an upgrade in the immediate future.

If you can't wait for me (or somebody else) to make an addon that will let you create custom lists and want to make your own single mailing list you could do as follows:
edit GuildMailer.lua like this:

edit line 104-116 to look like this:

function GuildMailer.makeList()
    return { "name1", "name2", "name3" }
end

the text on the button comes from line 9 in the GuildMailer.xml - in case you want to change that one aswell.