and other impatient Postfix mailers everywhere wrote:

If you are receiving larger numbers of reports about failing mail
transport from your customers these days, here is why.

Apparently, the nameservers for the and zones are overloaded (I get <10% ICMP_echo's
back, and virtually never any DNS answers), causing a more than
60-second connection-established -> 220 Hello banner
answering delay on typical Unix-based Sendmail (and other SMTP)
servers with default DNS resolver timeouts.

Perhaps this is related? (Server blocked off, rather than answering with
a query-denied, perhaps?)