[Exim] how to get rid of faked mail from own domain

Dear friends,

can one help me getting rid of messages like this:

(faked) sender emailaddress pretends to be local user
but is not sent by one of my (very few) mailservers


how can I check if the mail (with domain in my local_domains) is sent
by one of my mailservers ?

here is my acl:

acl_check_rcpt:

accept hosts = :

deny recipients = dfritsell@???

  deny    domains       = !+local_domains  : +relay_to_domains
          !verify       = sender/callout=30s

  deny  domains         = +local_domains
        !verify         = recipient/callout=10s, defer_ok, no_cache

  warn  message         = BAD_MAILHOST EBEWE Mail from not EBEWE
Mailserver i
        sender_domains  = !+local_domains  : +relay_to_domains
        condition       = ${if eq
{${hmac{md5}{exim@ebewe}{$body_linecount}}}\
                          {$h_X-Scan-Signature:} {0}{1}}
 warn message = X-Is-A-Bounce: 1
      senders = :

  deny    local_parts   = ^.*[@%!/|]

  accept  local_parts   = postmaster
          domains       = +local_domains : +relay_to_domains

  require verify        = sender

 accept  domains       = +local_domains
          endpass
          message       = unknown user
          verify        = recipient

  accept  domains       = +relay_to_domains
          endpass
          message       = unrouteable address
          verify        = recipient

  accept  hosts         = +relay_from_hosts

accept authenticated = *

# RBL

  deny    hosts = +rbl_hosts
                  message = host $sender_host_address is listed in
$dnslist_domain
                  dnslists = blackholes.mail-abuse.org :\
                         dialups.mail-abuse.org :\
                                         relays.mail-abuse.org :\
                                         dun.dnsrbl.net :\
                                         spam.dnsrbl.net :\
                                         relays.ordb.org

  deny    message       = relay not permitted

----------------------------------------------------------------------
Wolfgang Fuertbauer (E-Mail: Wolfgang.Fuertbauer@???)
EBEWE Pharma                          Tel: ++43 7665 8123 315
Mondseestrasse 11                     Fax: ++43 7665 8123 11
4866  Unterach, Austria
http://www.ebewe.com