The PAM module that sets and unsets environment variables is pam_env. This module allows the (un)setting of environment variables based on the rules specified in the configuration file /etc/security/pam_env.conf or an alternative file. The module can also read a file with simple KEY=VAL pairs on separate lines (/etc/environment by default) or a user-specific file ($HOME/.pam_environment by default). The pam_env module supports the use of previously set environment variables as well as PAM_ITEMs such as PAM_RHOST. The pam_env module should be the last one on the stack, as setting PAM environment variables can have side effects on other modules. The pam_env module provides the auth and session module types. References:

• pam_env - PAM module to set/unset environment variables
• pam_env(8) - Linux manual page

 Reverse DNS queries are used to find the domain name associated with an IP address. To perform reverse DNS queries, DNS servers use a special type of record called PTR (pointer) record. A PTR record maps an IP address to a hostname in the reverse lookup zone. For example, a PTR record for the IP address 185.230.63.186 would point to the hostname unalocated.63.wixsite.com. Reverse DNS queries are useful for validating email servers, troubleshooting network problems, and identifying malicious hosts. References:

• 2 Ways to Perform Reverse DNS lookups in Linux - howtouselinux
• Linux managing DNS servers (LPIC-2) · GitHub
• LPI Linux Certification/LPIC2 Exam 202/DNS - Wikibooks

Sieve is a language for filtering and sorting email messages on a mail server. Sieve core filters are the basic actions that can be applied to a message that matches a set of conditions. The following actions are available in Sieve core filters:

• discard: This action discards the message silently, without sending any notification to the sender or the recipient. This action is useful for deleting spam or unwanted messages.
• fileinto: This action delivers the message to a specified mailbox. The mailbox can be an existing one or a new one that is created by the action. This action is useful for organizing messages into different folders based on their content or headers.
• reject: This action rejects the message and sends a notification to the sender with a specified reason. The message is not delivered to the recipient. This action is useful for informing the sender that the message was not accepted due to some policy or error.

The other options are not valid actions in Sieve core filters. drop, relay, and fileinto are not recognized keywords by Sieve.

References:

• Sieve: An Email Filtering Language, section 2.10, “Actions”
• Sieve Tutorial, section 4, “Actions”