Have you looked at Outlook? If you've got one of the new tinkertoy office versions it may be omitted, but most people who have OE would have Outlook as well.
I don't use it, so I'm not really the one to give advice, but it does include the ability to use a "distribution list" and doesn't indicate any limit on number of addressees in Outlook Help.
Even if there is a limit, the ability to click on a list folder and send to everyone in the folder, rather than adding names individually, would make it much simpler to send to groups of xxx people by splitting your mail list into smaller separate dist lists, perhaps.
A disadvantage is that as it's described in the Help info, all the people on the list show in the "To:" box for all who receive it, which means that everyone who receives it has the email addy of everyone else who receives it. This is considered impolite by some and criminal by most recipients.
Someone more familiar with Outlook might be able to tell us whether a distribution list can be forced into the BCC: box where it should go.
There also appears to be a "mail merge" option in Outlook, that would address separate "mails" to each person in a "mailing list." Ability to port a mailing list to email is implied but the Help file doesn't give much detail. It might be obvious if/how it would work if I actually had a working account and address book set up in Outlook.