There is a detailed exposition of address classes and address
validation in https://www.postix.org/ADDRESS_CLASS_README.html#classes.
Instead of rephrasing that information multiple times in the
postconf(5) manpage, I'll just link to the defining text.
Wietse
The paste below is for senders; the recipient is case is similar.
smtpd_reject_unlisted_recipient (default: no)
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 considered "unknown" when 1) it does not match a
virtual(5) alias or canonical(5) mapping, and 2) the address
is not valid for its address class. For a definition of class-based
address validation, see ADDRESS_CLASS_README.html#classes.
This feature is available in Postfix 2.1 and later.
