www. subdomain required?

[strarsis]

@croessner, @onlime:
In the nginx configuration I noticed that there is also automx specific configuration for www. subdomain:
https://github.com/sys4/automx/blob/master/src/conf/nginx-automx.conf#L40
Is this also required additionally to the autoconfig subdomain?
How does the mail client know that www. subdomain is the next place to ask for the configuration?