[exim] authenticate_hosts "unknown" despite exim -bV listing…

Hi all,

I'm new to Exim, and having a bit of a problem. I want to authenticate with
my ISP's SMTP server, but so far no success.

exim -bV says (among other things):
Authenticators: cram_md5 plaintext spa

Yet, when I add authenticate_hosts = smtp.my.isp to the remote_smtp block
inside transports, running exim -bV also says:

2006-12-12 18:38:01 Exim configuration error in line 515 of
/etc/exim/exim.conf:
option "authenticate_hosts" unknown

Is there a clue in the house?

Thanks!
Jacob