Sounds like a scam, if you have any questions I'd call the dealership and speak to the salesperson. Most likely it's not them that sent the email to you.
I bought a vehicle in Ohio this past December 15 (I think) they gave me a Paper Plate good till January 31. Title came in about 2 weeks, I printed the Out of State Title form from IBMV. Had an Officer check the VIN and went to the IDMV and got the Plates & Title work done. Each State collects its own Sales Tax. This sounds like more Illinois BS to me. Tell them all correspondence needs to be by US Mail.