Some gmail gets through, some doesn't. Is there a time limit on the DNS check? A google search finds several timers, but nothing specific to DNS.
Log: Feb 17 06:18:10 mydomain postfix/smtpd[2619]: connect from unknown[209.85.219.177] Feb 17 06:18:10 mydomain postfix/smtpd[2619]: Anonymous TLS connection established from unknown[209.85.219.177]: TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits) Feb 17 06:18:10 mydomain postfix/smtpd[2619]: NOQUEUE: reject: RCPT from unknown[209.85.219.177]: 550 5.7.1 Client host rejected: cannot find your reverse hostname, [209.85.219.177]; from=<some.per...@gmail.com> to=<m...@mydomain.com> proto=ESMTP helo=<mail-yb1-f177.google.com> Feb 17 06:18:10 mydomain postfix/smtpd[2619]: disconnect from unknown[209.85.219.177] ehlo=2 starttls=1 mail=1 rcpt=0/1 bdat=0/1 quit=1 commands=5/7 Clearly the server is legit. https://bgp.he.net/ip/209.85.219.177 AS15169 IRR Valid 209.85.128.0/17 Google LLC
