Bulk mailing

Hi all,

I'm looking for suggestions on software or implementation of a (most
likely linux server based) bulk mailing solution.

We currently have a simple solution for news-letters; maintaining
subscriber lists, creating a fairly nice-looking message using a rich
text-editing widget and html-templates, and sending them out.

This solution is ASP-based, all the mail is sent out as part of a http
request, which works fine for ~200 subscribers. Problem is we're
aiming for a new client, a reasonably sized charity, with a subscriber
list of 50K and rising.

Can't really get our current solution to scale beyond a few 100s, so
we're looking for something that will let us drop the newsletter off
to another system which takes care of the bulk mailing part.
The current code is rather messy, so we're aiming for a rewrite in php
as well and deployment on a linux server.

I've done some looking into mailing lists, and figure that could work
if we find something which is either really simple to use, and will
let us create our own front end easily, or perhaps just rolling our
own.

Here I imagine something like a perl/python script, which is run from
the crontab every 15 min, queries the database for new mail to be
sent, mark it as processed to prevent the script from going off again
should the job take more than 15 min, then start fetching subscriber
data, sorted by domain or something,, so we can add up to ~20 bcc's to
each message we're sending out.

But there's the question of dealing with errors and retries and such,
so I'm sure we'd be best off with a pre-made solution.