Sound advice.
Normally I'd always opt for permission based email marketing, but seeing as these were gathered professionally and its for charity I'm sure you'll be fine. You have to be absolutely certain these people are potentially interested in your email. It all depends on the delivery though....
Like Dan says, make it as easy as possible to opt out of further email communications. Make it clear this will be a one off unless they are interested in more information.
Don't push any blatant advertising messages. Your taking some of their time, give them something of value to them in return.
If a lot of these email addresses are Hotmail and Gmail based you need to think carefully about the title as well. Chances are, you'll end up in their spam filter. A title written completely in caps to draw attention will be read as 'SPAM', so try and show them this is not spam and who it is actually from.
Hope that helps..