The reason has to do with the SSL certificate. It is unlikely that you want to purchase an SSL cert for all "autodiscover." SMTP domains you host. Using a second IP address gives you the ability to create a separate website that does not use SSL.
So your second autodiscover url would be:
The autodiscover.xml would then be set to redirect to:
This way you only need an SSL cert for autodiscover.mainsmtpdomain.com.
When searching for the autodiscover service a client will follow the following in this setup scenario for your Second SMTP domain:
- Tries https://domain2.com/autodiscover/autodiscover.xml (Fails)
- Tries https://autodiscover.domain2.com/autodiscover/autodiscover.xml (Fails)
- Tries http://autodiscover.domain2.com/autodiscover/autodiscover.xml (succeeds)
If the third option were to fail it would go on to look for SRV records (_autodiscover._tcp.mainsmtp.com)
You can not add a new virtual directory to the CAS default website to accomplish this. This is because you would then have two "autodiscover" virtual directories in the same site.
You could create a new site on the CAS and use the above outlined steps to accomplish this. However, this also would require you to set the default website to use a specific IP and add an additional IP to your CAS NIC for the new site. See the Autodiscover Whitepaper: Scenario 4