External address not valid for deadline notification???

[Michael Pokraka]

Hi all,
On a 4.6c system, I'm unable to use an email address as a deadline
notification recipient, I get "Expression .... has an invalid type or
reference".
The same expression works for a send mail step, so I could use a
modeled deadline - but I'd prefer to avoid that if possible.
 
Is there something simple I'm missing or any trick to get this to work
as a standard deadline notification? (Forwarding the inbox is an
alternative which I'm also investigating - not a good outlook as there
are too many agents involved).
 
Any input greatly appreciated,
Cheers
Mike