How about using using aliases? It works beautifully for smaller lists. I've never used 400, but I can't imagine why it wouldn't work.
If you have a comma delimited list of addresses, you could just paste them into /etc/aliases in this form:
alias_name: name@xxxxxxxxxxxx, name@xxxxxxxxxxxx, ...
You must run "newaliases" after you edit the file, then just send one email to aliases@yourmailhost and all the defined addresses get it.
man aliases
If you do take this approach, I recommend sending to: your email and bcc: to the alias name. This way if anyone does a reply-to-all it won't cause a massive reply explosion to everyone on the list.
