Symptoms
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A mail tester mail-tester.com or some similar resource shows the following result after an email message from Plesk is sent to it:
Testing generated the following problem:
Delivered to external 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:
Reverse DNS does not contain the hostname
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Emails are not delivered to public mail servers, such as Hotmail and Gmail.
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The server has a PTR record configured, but this PTR record does not comply with FCrDNS (host in reverse DNS does not point back to IP):
# host 203.0.113.2
2.113.0.203 in-addr.arpa domain name pointer 203-0-113-2.example.com# host 203-0-113-2.example.com
203-0-113-2.example.com 203.0.113.3
Cause
PTR record cannot be forward-confirmed, and the mail is rejected as per RFC7601 Section 3.
Resolution
Contact hosting provider or ISP to change PTR record of the IP address.
This record may only be modified by the IP address owner.
The PTR record must have a full chain and may point to server's hostname, or to one of the server's domains:
# host 203.0.113.2
2.113.0.203 in-addr.arpa domain name pointer 203-0-113-2.example.com
# host 203-0-113-2.example.com
203-0-113-2.example.com 203.0.113.2