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One or more of the following symptoms may apply:
Domain mail test at https://www.mail-tester.com/ or some similar resources show the following output:
CONFIG_TEXT: Testing generated the following problem:
Delivered to internal network by a host with no rDNS
This may be a false-positive, please check the reverse DNS test below to confirm or not this issue
Your reverse DNS does not match with your sending domain.
Reverse DNS lookup or reverse DNS resolution (rDNS) is the determination of a domain name that is associated with a given IP address. Some companies such as AOL will reject any message sent from a server without rDNS, so you must ensure that you have one. You cannot associate more than one domain name with a single IP address.
Your IP address 203.0.113.2 is associated with the domain 203.0.113.2.example.com. Nevertheless your message appears to be sent from vps.example.org. You may want to change your pointer (PTR type) DNS record and the host name of your server to the same value.
Checking the same domain on mxtoolbox.com shows:
CONFIG_TEXT: Reverse DNS does not contain the hostname
Emails are not delivered to public mail servers, such as Hotmail and Gmail
The server has a PTR record configured, but this PTR record does not comply with FCrDNS (forward-confirmed reverse DNS)
# host 203.0.113.2
22.214.171.124 in-addr.arpa domain name pointer 203-0-113-2.example.com
# host 203-0-113-2.example.com
PTR record cannot be forward-confirmed, and the mail is rejected as per RFC7601 Section 3.
Contact hosting provider or ISP to change PTR record of the IP address.
The PTR record of the IP address should point to server's hostname (Tools & Settings > Server Settings > Full hostname), or to one of the server's domains.