When you create a site (with the www as default) ISPConfig create the www.domain.com
host and the domain.com alone.
Email must works.. even when everywhere you will see firstname.lastname@example.org
, its a "bug" "missconfig" (i dont know, look for previous posts).
If you dont want a www site (for example, im using some sites just for email, www are hosted over windows servers) put something like "mail", it works, then, create a codomain www.domain.com
with your www server ip to create a dns record. Its look like the hostname field must be filled.