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PTR checks the domain IP address to validate sender address.
But against what? the SMTP HELO hostname?
Or against email domain?
for example if I send an email from [email protected] from a SMTP server with domain domain2.com
Is PTR check performed for domain1.com or domain2.com?

Shared Hosting panels(e.g. CPanel) documentation says only set PTR for hosts main domain and not for every vhost.
in this case checking against email's domain will fail But this also means anyone can send email for any vhost (also vhosts which are not on my NS) if their server has correct PTR for itself but not for vhost

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PTR is the short form of the word pointer. This assigns a domain to a specific IP address - or in other words: the PTR record "points" to the IP address.

This process is known as reverse mapping, because the query is usually done in reverse. The check will be performed for the domain of the mail adress (domain1.com) and therefore, will give an error -> because the IP adress will not be valid (points to another domain).

Correctly set reverse mapping is absolutely essential for reliable operation of a mail server.

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  • So why shared hosts only set server's hostname PTR and not for each vhost? is this incorrect?
    – Ariyan
    Feb 23, 2020 at 21:24

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