[Exim] Understanding acl-check_rcpt

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I would appreciate some help to understand the process please.

I have a lot of frozen messages like this on two newly build mail
servers (Debian Woody with Exim 4.33):
========
Message 1BQMZU-0006Sj-Ma has been frozen (delivery error message).
The sender is <>.

The following address(es) have yet to be delivered:
kxjvx@???: SMTP error from remote mailer after RCPT
TO:<kxjvx@???>: host mail3bb.sun.ac.za [146.232.128.106]: 550
unknown +user
=========

And I have the following in my acl_check_rcpt:

================

accept hosts = :


Then three warn conditions and one defer condition which concerning
the different user preferences concerning spam

Then:

  deny    domains       = +local_domains
          local_parts   = ^[.] : ^.*[@%!/|]
          message       = restricted characters in address

  deny    domains       = !+local_domains
          local_parts   = ^[./|] : ^.*[@%!] : ^.*/\\.\\./
          message       = restricted characters in address

  deny local_parts = ^[a-zA-Z0-9.]+-outgoing
           domains = maties2.sun.ac.za : sun.ac.za
         condition = ${if eq {$sender_address}{majordomo@???}{no}{yes}}
          message  = Message viewed as an effort to bypass majordomo controls.  \
                     Please contact sysadm@??? if you do not agree.

  accept local_parts = postmaster
         domains = +local_domains

deny message = Sender verification failed
       hosts = !(network segment)
       !verify = sender

deny    hosts = (list of hosts)

 deny message = sender envelope address $sender_address is locally blacklisted here. If you think this is wrong, get in touch with postmaster
       !acl = acl_whitelist_local_deny
       senders = ${if exists{CONFDIR/local_sender_blacklist}\
                             {CONFDIR/local_sender_blacklist}\
                             {}}

  deny message = sender IP address $sender_host_address is locally blacklisted here. If you think this is wrong, get in touch with postmaster
       !acl = acl_whitelist_local_deny
       hosts = ${if exists{CONFDIR/local_host_blacklist}\
                             {CONFDIR/local_host_blacklist}\
                             {}}

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

  deny    message = unrouteable address
          recipients = *sun.ac.za : \
           *@dbm;/etc/exim4/relaydomains.db : dbm;/etc/exim4/relaydomains.db
         !verify = recipient

  accept domains = +relay_to_domains
         endpass
         message = unrouteable address
         verify = recipient/callout=20s,defer_ok

accept hosts = +relay_from_hosts

  require verify        = sender

accept authenticated = *

deny message = relay not permitted

==============

Now my question: Why did "verify recipient" and "require verify =
sender" not cause this message to be denied at smtp-level?

Regards
Johann
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Johann Spies          Telefoon: 021-808 4036
Informasietegnologie, Universiteit van Stellenbosch

     "But as it is written, Eye hath not seen, nor ear
      heard, neither have entered into the heart of man, the
      things which God hath prepared for them that love
      him."      I Corinthians 2:9
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