[exim] One of my boxes can't send mail

One of my boxes which was working a few days ago has stopped sending
mail. I found this test in the exim wiki:

catherine ~ # exim -bt michael
michael@??? is undeliverable: Unrouteable address
    <-- michael@???

Here's a more complete attempted transaction:

catherine ~ # telnet localhost 25
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
220 catherine.espersunited.com ESMTP Exim 4.67 Tue, 15 May 2007 11:00:49
-0500
helo test
250 catherine.espersunited.com Hello localhost [127.0.0.1]
mail from: michael@???
250 OK
rcpt to: michael@???
550-Verification failed for <michael@???>
550-Unrouteable address
550 Sender verify failed
}
500 unrecognized command
quit
221 catherine.espersunited.com closing connection
Connection closed by foreign host.

Here's /etc/exim/exim.conf. I've removed the comments for conciseness:

domainlist local_domains = @ espersunited.com :
catherine.espersunited.com
domainlist relay_to_domains =
hostlist relay_from_hosts = 127.0.0.1 : 70.234.122.251 :
70.234.122.250 : 70.234.122.248

acl_smtp_rcpt = acl_check_rcpt
acl_smtp_data = acl_check_data


qualify_domain = catherine.espersunited.com


never_users = root


host_lookup = *
rfc1413_hosts = *
rfc1413_query_timeout = 5s

ignore_bounce_errors_after = 2d

timeout_frozen_after = 7d


begin acl

acl_check_rcpt:
  accept  hosts = :
  deny    message       = Restricted characters in address
          domains       = +local_domains
          local_parts   = ^[.] : ^.*[@%!/|]
  deny    message       = Restricted characters in address
          domains       = !+local_domains
          local_parts   = ^[./|] : ^.*[@%!] : ^.*/\\.\\./
  accept  local_parts   = postmaster
          domains       = +local_domains
  require verify        = sender
  accept  hosts         = +relay_from_hosts
          control       = submission
  accept  authenticated = *
          control       = submission
  require message = relay not permitted
          domains = +local_domains : +relay_to_domains
  require verify = recipient
  accept
acl_check_data:
  accept
begin routers
dnslookup:
  driver = dnslookup
  domains = ! +local_domains
  transport = remote_smtp
  ignore_target_hosts = 0.0.0.0 : 127.0.0.0/8
  no_more
system_aliases:
  driver = redirect
  allow_fail
  allow_defer
  data = ${lookup{$local_part}lsearch{/etc/mail/aliases}}
  file_transport = address_file
  pipe_transport = address_pipe

userforward:
driver = redirect
check_local_user
file = $home/.forward
no_verify
no_expn
check_ancestor
file_transport = address_file
pipe_transport = address_pipe
reply_transport = address_reply



localuser:
driver = accept
check_local_user
local_part_suffix = +* : -*
local_part_suffix_optional
transport = local_delivery
cannot_route_message = Unknown user



begin transports
remote_smtp:
driver = smtp

local_delivery:
driver = appendfile
directory = /home/$local_part/.maildir
maildir_format
delivery_date_add
envelope_to_add
return_path_add


address_pipe:
driver = pipe
return_output


address_file:
driver = appendfile
delivery_date_add
envelope_to_add
return_path_add


address_reply:
driver = autoreply



begin retry

*                      *           F,2h,15m; G,16h,1h,1.5; F,4d,6h
begin rewrite

begin authenticators


I have tried everything I can think of to get this to work, even just so
that it will mail out my logwatch reports, but nothing's worked. Can
anyone help me? This box runs Gentoo with kernel 2.6.19-gentoo-r5 SMP,
has public IP address 70.234.122.251, and exim-4.67 with the following
USE flags:

catherine exim # emerge -pv exim

These are the packages that would be merged, in order:

Calculating dependencies... done!
[ebuild R ] mail-mta/exim-4.67 USE="X exiscan-acl ldap mysql pam
perl sasl ssl syslog tcpd -dnsdb -domainkeys -dovecot-sasl -exiscan
-gnutls -ipv6 -lmtp -mailwrapper -mbox -mbx -nis -postgres -radius -spf
-sqlite -srs" 0 kB