[Exim] exim as smtp-auth client does not recognize server's …

hello,
i am new to exim but i managed to configure it so that it does what i
want except one last problem:

i have no clue why problem occurs, after searching the net for two days
and reading the exim spec a dozen times i am really stuck.

it seems that exim (4.20 under cygwin) does not recognize the advertised
auth methods of the smarthosts i configured to use to send incoming mail
to (mail.gmx.de, smtp.web.de, smtp.1und1.de). so i want to use exim as
an smtp client in this situation.

if the transporter (remote_smtp) is configured with hosts_try_auth the
corresponding smtp server reply with an error like this:
"530 Authentication required"
"Need to authenticate via SMTP-AUTH-Login"
if i use hosts_require_auth exim complains the following:
"authentication required but server did not advertise AUTH support"

i checked the log and the servers exim speaks to correctly advertise
their supported auth methods. i also telnetted to them and the reply to
ehlo seems fine to me.

the authenticators should be configured correctly as there should not be
much that could go wrong i hope:

login:
      driver = plaintext
      public_name = LOGIN
      server_prompts = "Username:: : Password::"
      client_send = ": <user> : <pass>"

plain:
      driver = plaintext
      public_name = PLAIN
      client_send = "^<user>^<pass>"

cram_md5:

      driver = cram_md5

      public_name = CRAM-MD5

      client_name = "<user>"
      client_secret = "<pass>"

i have no idea what i am missing.
thanks a lot in advance for any hints,
bjoern