The server has two network interfaces: eth0 with direct IP = xxx.xx.xx.18 connected to provider, and eth1 with another direct IP = xxx.xx.xx.25
DNS for the zone is configured correctly and it associates the latter IP with the domain name = name.org
Emails sent to various online email services (incl. yahoo.com) are successfully delivered, but Gmail rejects emails with the following error:
<<< 550-5.7.1 [xxx.xx.xx.18] The IP you're using to send mail is not authorized to <<< 550-5.7.1 send email directly to our servers. Please use the SMTP relay at your <<< 550-5.7.1 service provider instead. Learn more at <<< 550 5.7.1 https://support.google.com/mail/?p=NotAuthorizedError y76-v6si291845lfa.11 - gsmt p 554 5.0.0 Service unavailable
My Internet provider says it configured PTR at their servers to point to my IP, which corresponds to my domain name, (i.e. xxx.xx.xx.25). They say it should help. So, the problem could be because Gmail sees my emails sent from xxx.xx.xx.18, but I can't understand why? xxx.xx.xx.18 is not resolved to my domain name (name.org).
Any ideas? Thanks in advance!