"Send As" permissions on a group - Exchange 2010

[stoofa]

OK - this one is now doing my head in!

I have created a "Mail Enabled Security Group" called IT Department.
Under a user called "Terry Test" I have given him the "Send As" permission on this newly created group.
I've waited over the 2hrs (cache) time.
Have attempted to set the "From" address to the group in question and send a test message.
Message fails with:

You can't send a message on behalf of this user unless you have permission to do so........

This user has got the "Send As" permissions for this group.
So what am I doing wrong?
I'm missing something obvious but I can't find it......

 

[stoofa]

I was under the impression that if you use:

"Send on behalf" the "From" address would read:
John Smith sent on behalf of IT Department

Whereas using the "Send As" permission the "From" address would simply read:
IT Department

As a Domain Administrator I can send as anyone - so I can happily put "From" IT Department and it just works.

However regular users cannot, even though I make them a member of the IT Department group and give them the "Send As"