If you're outlook is connected to an exchange server, then you're in a tough spot since exchange allows a total of 32Kb for recipient lists or rules. Sounds wacky, but it's true. The only option you really have is to try (it also may not work) to create an email distribution list that holds all the addresses you need, keeping in mind that the 32k limit still holds. If you're not on an exchange server, then I'd still go the router of creating a distribution list and send it like that, however, if you're relaying email through an ISP as opposed to hosting your own email, then you risk being tagged as a spammer. I'd probably start by asking your IT department (if there is one available) to see what kind of services are available for that.
The only other option that I could think of would be a 3rd party mailer like Constant Contact
, that a few of my clients use and it really works for them.