[exim] notquit acl, and variables

I have a situation which I'm failing to understand.

In a nested acl called during RCPT I'm setting a variable
and returning "defer". I'm logging a tmp-reject. The initiator
says RSET and drops the connection (I suspect it's Exchange
Server). In the notquit acl I try to use the variable;
it appears empty.

Both _m and _c variable types do this. Comments, anyone?

Cheers,
     Jeremy