domain_literals_not_allowed

[#1]

What does the error message "domain_literals_not_allowed" mean from the plus.net email server?
One of my email accounts is trying to send periodic emails to my email account at plus.net but it's getting rejected.
The log file entry says:
May  8 03:45:51 server qmail: 1210232751.151469 delivery 4556: failure: Connected_to_84.92.2.1_but_sender_was_rejected./Remote_host_said:_501_<#@[]>:_domain_literals_not_allowed/
The email was sent to the address postmaster@hostname.domain.com which is setup to automatically redirect to email-username@plusnet-hosted-domain.com.
Any ideas what's wrong here?

[#2]

I think domain literals are in the format user@[Ip.Address] rather than user@domain.com.  I wonder if you have an address set somewhere (such as the from address, reply to etc...) that is a domain literal that it isn't liking.
Phil

[#3]

Well I just sent an email from my gmail account to:
postmaster@hostname.domain.com
Which redirects straight to:
email-username@plusnet-hosted-domain.com
And it bounced back. No user@ipaddress is used in the chain. Is it possible the plus.net platform doesn't like accepting emails from hostname.domain.com addresses? The hostname.domain.com is the name of the email server which reverses the server IP address resolves to.
The error message in bounce was:
<email-username@plusnet-hosted-domain.com>:
handlers permanentfail

[#4]

The plus.net server is actually rejecting all email forwarding requests regardless of the domain/subdomain. I'll test redirects to other email services to see if the problem persists everywhere (and is mine to solve).

[#5]

Hi there,
Just wanted to let you know I have noticed this thread and am going to keep an eye out for the results of your testing to other email services to see where the problem lies.