It is industry standard for ISP's and mail service providers to limit the amount of messages that can be sent out at any one time to around 250 (give or take).
Spam laws have tightened (99% of all mail that is sent is now spam!) and domains - and the providers that host them - get blacklisted across the Internet if regulations are violated.
As a result, all major email suppliers insist that customers use an authenticated bulk mailing service to send bulk mails. Bulk mails don't go out from "your mailserver" (although some can create the illusion that they do), they go out from a server that is authorised to send out bulk mail and which adheres to the industry regulations and data protection etc.
There are a number of suppliers that offer a free service for low volume bulk mailings. You just set up an account, create a contact list, upload it, create your mailer/newsletter/circular and send it out. These kind of systems are easier than Outlook etc anyway as they are built for specifically what you want to do. You can create different groups and send newsletters out to all or specific ones. You also get to see valuable data such as who's opened it and who's clicked on any links in your email (although you might have to upgrade to a paid plan for that).
NB: It's irrelevant that your recipients actually want to hear from you. For example, a golf club who wanted to email their 800 members (who really want to hear from the secretary!) would have to use a bulk mailing service.
MailChimp is a very easy to use bulk mailing platform. This explains how you can send out mail using Mailchimp.
Create your account (or we can create this for you) and then login with your Username and Password at: http://login.mailchimp.com.
When you have logged in:
If you want to add more subscribers to your list, when you first log in, click "Lists" from the menu to the left and click the icon:
When you have inserted the details, click Subscribe.
If you need help setting up your account or sending out your first newsletters, let us know.