exim - Could not complete sender verify callout

[ddaas]

Hi there,
I am new to exim so please excuse if it is a silly question. I am not new to Linux or Network Administration in general.
I need a fast solution because I have a server where users can't receive their e-mails. Any help is really really appreciate.

The server (FC4 + cpanel) runs exim 4.52
The configuration is standard, no special things. It wasn't configured by me, but now I am in charge..
There are a lot of mails in exim_reject rejected with: Could not complete sender verify callout

2006-03-26 04:11:54 H=(mail.xxx.com) [x.x.156.26] F=<xxxx@tips.xx.xx.xx> temporarily rejected RCPT <yyy@xxx.com>: Could not complete sender verify callout

There are also authentic domains (ex: mail.citigroup.com ) for which emails are rejected.
I've read something about exim verifications and I understood how the whole thing (callout verification) works: the server tries a smtp connection with the sender server and if something goes wrong the message is rejected. This is mainly because of wrong configured servers and MX records. It I am wrong please correct me.

What I want to do, is to configure the server to exclude some domains which are not spam-senders.
How can I accomplish that?
I've read about sender_verify, sender_verify_hosts *, sender_verify_hosts_callback *, sender_verify_callback_domains ! *.some.domain :* but I don't know where in exim.conf to set them. When I restart exim after I add these option I get an error:

Starting exim-smtps: 2006-03-27 22:50:56 Exim configuration error in line 29 of /etc/exim.conf:
main option "sender_verify_callback_domains" unknown

Please tell me how can I exclude some domains from this verification, and where in exim.conf to add the new options.

Thanks for your help

[ddaas]

HI,
I created an acl. Something like:

#added just for testing on 28-03-2006
test_acl:
accept hosts = mail.xxx.com

Then before the line: require verify = sender/callout (the only line which refers to callout verifying), I added:

condition = ${if !match {$sender_address_domain}{+test_acl}}

There is no configuration error, but also nothing has changed.

From the logs I get:

2006-03-28 12:44:24 H=(mail.xxx.com) [xx.xx.156.26] F=<dxrxha@txxxl.com> temporarily rejected RCPT <xxx@stxxx.com>: Could not complete sender verify callout

What have I done wrong? What to do next?
I will read and learn about exim, about principles and what happens under the hood, but in the moment I need a fast solution so that my clients can get their mails.

Thanks a lot.