[exim] Sudden failure accepting incoming email, with "Unrout…

I've been hit with an unexpected failure of Exim to accept my local email
twice in the past week.

Environment: Exim 4.72, MacOS X 10.5.x
and an Exim config that's been happily running for a couple of years (and
been few a through upgrades of Exim version before that)

Suddenly, seemingly out of the blue, the mainlog shows "no IP address found
for" (the sending host, my MX) and rejection of my local address with
"Unrouteable address".

(I've adjusted email addresses, just because of spam, but domains and
everything else are the same. Just in case there's a DNS snafu in there that
someone can see.)

2012-10-22 20:24:47 no IP address found for host dm.gbnet.net (during
SMTP connection from [194.70.142.30])
2012-10-22 20:24:47 H=(dm.gbnet.net) [194.70.142.30]
F=<opennms-discuss-bounces@???> rejected RCPT
<jrg@???>: Unrouteable address

(prior to that, successfully accepted emails were appearing just fine in the
log, and my MX's IP address was resolving ok.)



1. Just restarting Exim (as I did after the first failure, when I saw it)
seems to clear the problem. But after I first had the problem last week,
restarting only seemed to fix it until it broke again on the 22nd (and I
only just noticed it had broken, again)

It clearly seems like it's something about name resolution, because the
inability to resolve the sending host appears at the same time. The local
DNS server seems fine, but what might make the resolver suddenly break?


2. Why is my Exim config not just accepting my local email anyway? It's in
my local_domains list. Is there something I'm missing, to make it more
robust to failures of DNS?

domainlist local_domains = ...
     watching.org : \
     ...

check_recipient:
   accept  hosts = :
   deny    senders = @@lsearch;/etc/mailreject
   deny    message = unrouteable address
          !verify = recipient
   accept  domains = +local_domains
   accept  hosts = +relay_hosts
           control = submission
   accept  authenticated = *
           control = submission
   deny    message = relay not permitted

and here's my directors from my config:

lookuphost:
driver = dnslookup
domains = ! +local_domains
no_search_parents
transport = smtp

literal:
driver = ipliteral
domains = ! +local_domains
transport = smtp
self = pass
no_more

system_aliases:
driver = redirect
allow_defer
allow_fail
data = ${lookup{$local_part}dbm{/etc/aliases.db}}
file_transport = address_file
pipe_transport = address_pipe
retry_use_local_part
domains = +local_domains
user = exim

userforward:
driver = redirect
allow_filter
check_local_user
domains = +local_domains
file = $home/.exim-forward
file_transport = address_file
pipe_transport = user_address_pipe
reply_transport = address_reply
no_verify

localuser:
driver = accept
check_local_user
transport = local_delivery
domains = +local_domains

fallback:
driver = redirect
allow_defer
allow_fail
data = jrg@???
domains = watching.org
retry_use_local_part