sendmail can not deliver to gmail - IPv6 sending guidelines regarding PTR records not met

Solution 1:

From Google's point of view, they are trying to verify the identity of the IP address connecting to them, so they will attempt to look up the PTR record for 2a01:4f8:212:27c8::2.

When they resolve that to staging.findix.com, they will then attempt to check that this resolves back to 2a01:4f8:212:27c8::2 - which it doesn't - see this lookup result.

Reverse (PTR record) Lookup

cwatson@thor:~$ nslookup 31.220.4.52
Server:     127.0.0.1
Address:    127.0.0.1#53

Non-authoritative answer:
52.4.220.31.in-addr.arpa    name = tyr.vikingserv.net.

Forward (A record) Lookup

cwatson@thor:~$ nslookup tyr.vikingserv.net
Server:     127.0.0.1
Address:    127.0.0.1#53

Non-authoritative answer:
Name:   tyr.vikingserv.net
Address: 31.220.4.52

Solution 2:

1. Check the current protocols:

   postconf inet_protocols

   net_protocols = all

2. Edit cf File if it returns all

   grep 'inet_protocols' /etc/postfix/main.cf

   inet_protocols = ipv4

3. Restart

   service postfix restart

4. Check it again

   postconf inet_protocols

   inet_protocols = ipv4

Solution 3:

Your smtp server hostname could be a problem. It seemed to be the case for me. POstfix was somehow sending a host name to smtp.gmail.com not relevant with my smtp domain name.