My mail provider allows me to use distribution lists with up to 500 members but when I use Outlook 2007, I can only send to about 100 people. Is there any way I can send more than 100 emails from distribution list?

No, you can't use larger distribution lists with Outlook if Outlook says "No". Outlook 2007 SP2 removes a limit from the size of the DL, but there is a limit to the number of addresses you can put in the TO/CC/BCC fields when you send using a SMTP server. The workaround: split the DL and use Resend this message to send it to the additional DLs.

To Split a DL

Split the DL: Open the DL and use the Forward to internet command to create a text file of the DL. Delete some names from the DL to get under 100 and save it. Open the text file on the Forward message and copy the addresses you deleted from the other DL. Paste them into the Members field of a new DL. Name it 'dl-name 2' and Save.

To Resend a Message

To use: Send the message to DL #1, then open the sent message and choose Resend this message. Delete the names in the To field and replace with DL #2.

Alternately, compose the message, save to drafts and either hit Forward on the draft to send to the first DL or Ctrl+C, V to make a copy of it then open each copy and address it with one of the DLs then send. Note: When you use this method, the Sent item might be placed in Drafts.

If you need more complete instructions, screen shots, or tutorials, see:
How to Forward a Distribution list and Create DL from list of addresses