Domain Name for Accessing Site and in Email Notification are Different

I've recently began to use a new domain name I purchased outside forumotion, www.blackorder.web.id. Things are fine, and everyone can access the forum via the new address, but there is one problem.

The forum sends email notifications of new replies, but the forum address they use here is not www.blackorder.web.id, but blackorder.web.id (without the 'www'), which doesn't work for the forum. It's a small matter, but it really makes accessing new replies through emails inconvenient.

I tried removing the domain name and assigning it to the forum again, but this doesn't change anything. Does anyone have a solution?