Postfix's documentation defines smtpd_reject_unlisted_recipient (default: yes) as
Request that the Postfix SMTP server rejects mail for unknown recipient addresses, even when no explicit reject_unlisted_recipient access restriction is specified. This prevents the Postfix queue from filling up with undeliverable MAILER-DAEMON messages.
An address is always considered "known" when it matches a virtual(5) alias or a canonical(5) mapping.
The recipient domain matches $mydestination, $inet_interfaces or $proxy_interfaces, but the recipient is not listed in $local_recipient_maps, and $local_recipient_maps is not null. The recipient domain matches $virtual_alias_domains but the recipient is not listed in $virtual_alias_maps. The recipient domain matches $virtual_mailbox_domains but the recipient is not listed in $virtual_mailbox_maps, and $virtual_mailbox_maps is not null. The recipient domain matches $relay_domains but the recipient is not listed in $relay_recipient_maps, and $relay_recipient_maps is not null. This feature is available in Postfix 2.1 and later.
and smptd_reject_unlisted_sender (default: no) as
Request that the Postfix SMTP server rejects mail from unknown sender addresses, even when no explicit reject_unlisted_sender access restriction is specified. This can slow down an explosion of forged mail from worms or viruses.
An address is always considered "known" when it matches a virtual(5) alias or a canonical(5) mapping.
The sender domain matches $mydestination, $inet_interfaces or $proxy_interfaces, but the sender is not listed in $local_recipient_maps, and $local_recipient_maps is not null. The sender domain matches $virtual_alias_domains but the sender is not listed in $virtual_alias_maps. The sender domain matches $virtual_mailbox_domains but the sender is not listed in $virtual_mailbox_maps, and $virtual_mailbox_maps is not null. The sender domain matches $relay_domains but the sender is not listed in $relay_recipient_maps, and $relay_recipient_maps is not null. This feature is available in Postfix 2.1 and later.
I find the concept of an unlisted sender or recipient confusing. My mail server, which was spun up with iRedMail, set both to yes. They both seem to share the same criteria, so it leads me to believe that Postfix acknowledges something as "verified" if it is considered "known", which follows its given definitions of what is considered "known". However, my mail server works fine, yet I can't imagine that if I sent an email from my gmail account to my mail server, that it would pass the critieria as being "known" and would reject it. I've looked at my domains, forwardings and mailbox tables iRedMail sets up, and my domain is the only one listed for domains, my forwardings only show the users on my domain including the postmaster, and I'm honestly not sure what to make of mailbox but I don't think it's key to understanding this. I also have no /etc/postfix/virtual folder and no /etc/postfix/canonical folder, so how can my email be considered listed and therefore not rejected?
Regardless, I think I can rest assured that my gmail account wouldn't be listed as "known", so why is not an unlisted sender and therefore can't send mail to my MDA?
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