I agree with Angelle, same server or behind same firewall/router would be a problem. If router is attacked, no traffic gets through.
An email server can be hosted completely offsite from where the book/webserver is located. Also, some services like Homepagenames can route all email for your domain to a yahoo/hotmail account. For example: anybody@hollywood.com could go to hollywood@yahoo.com
In this made up example, the email would never even touch the book and go right to yahoo.