basic spam filtering includes rejecting mail from servers without reverse DNS. Zimbra is a mail server company. Should know better.
Dec 30 03:16:19 foo postfix/smtpd: NOQUEUE: reject: RCPT from unknown[188.8.131.52]: 450 4.7.1 Client host rejected: cannot find your reverse hostname, [184.108.40.206]; from=
<email@example.com> to=<firstname.lastname@example.org> proto=ESMTP helo=<web3.zimbra.com>
because of that I didnt receive an important annoucement about this 0-day. And I cant recover my regular username's password I've used for 8+ years.