Earthlink spamblocking possible to whitelist an entire domain?

[elmwood]

I run a message board dealing with urban planning, using vBulletin - the same collection of scripts as the Straight Dope Message Board.

Many users will subscribe to threads and turn on private message notification. Unfortunately, many of those users are Earthlink subscribers, and the notification messages ALWAYS bounce back to me with the following message:

I apologize for this automatic reply to your email.

To control spam, I now allow incoming messages only from senders I have approved beforehand.

If you would like to be added to my list of approved senders, please fill out the short request form (see link below). Once I approve you, I will receive your original message in my inbox. You do not need to resend your message. I apologize for this one-time inconvenience.

Click the link below to fill out the request:

It’s board policy to NEVER “click on the link below.”

Anyhow, one of my users says it’s impossible to whitelist an entire domain; it’s only possible to whitelist individual addresses. Thus, you can’t whitelist everything coming in from example.com; just those of the addresses you chose. True?