An ISP I use recently switched DNS servers and this, it turns out, broke sendmail. Running
echo 'check_mail cfsc-bounces@computingfrontiers.org' | sdmail -d8.20 -bt
gave
dns_getcanonname(computingfrontiers.org, trymx=1)
dns_getcanonname: trying computingfrontiers.org. (AAAA)
NO: errno=0, h_errno=1
dns_getcanonname: trying computingfrontiers.org.mscd.edu (AAAA)
NO: errno=0, h_errno=1
which is not good. Adding
O ResolverOptions=WorkAroundBrokenAAAA
to sendmail.cf fixed things
dns_getcanonname(computingfrontiers.org, trymx=1)
dns_getcanonname: trying computingfrontiers.org. (AAAA)
NO: errno=0, h_errno=4
dns_getcanonname: trying computingfrontiers.org. (A)
YES
The sendmail change notes say
New ResolverOptions setting: WorkAroundBrokenAAAA. When
attempting to canonify a hostname, some broken nameservers
will return SERVFAIL (a temporary failure) on T_AAAA (IPv6)
lookups. If you want to excuse this behavior, use this new
flag. Suggested by Chris Foote of SE Network Access and
Mark Roth of the University of Illinois at
Urbana-Champaign.
which means someone will have to fix the DNS server...
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