250 doesn't exceed any OE limitations, but most ISPs won't allow it.

As Frank said, not an Outlook Express limitation, but likely an ISP limitation.

You might want to look into a list server.  As an example, see http://groups.yahoo.com/ 
You have a number of options with these lists.  You can either manage the members yourself
or let people add/remove themselves from the list.  You can also control whether any
member of the list can send messages to all members or allow only yourself to send the
messages.