A theory of the problem: When sqlite.org is sending email, it might send the message from either of two different IP address: * ipv4: 126.96.36.199 * ipv6: 2600:3c00::f03c:91ff:fe96:b959 The metrics provided to me by [Google Postmaster Tools] show that 188.8.131.52 has the best possible "IP Reputation", but that 2600:3c00::f03c:91ff:fe96:b959 has the worst possible IP Reputation. The bad IP Reputation for 2600:3c00::f03c:91ff:fe96:b959 appears to be because Gmail is grouping all traffic from any 2600:3600:: IP address into a single pool. At least, that is what [Google Postmaster Tools] is telling me. It is as if they are only considering the first 32 bits of the 128-bit IP address. : https://gmail.com/postmaster/ I have adjusted the configuration of [postfix] on sqlite.org so that it now only sends email via IPv4. Perhaps this will help. : http://www.postfix.org/ If you have alternative suggestions, please speak up.