Нашёл все эти сообщения в файле: /etc/Postfix/bounce.cf.default
# Do not edit this file. This file shows the default delivery status
# notification (DSN) messages that are built into Postfix.
#
# To change Postfix DSN messages, perhaps to add non-English text,
# follow instructions in the bounce(5) manual page.
#
#
# The failure template is used when mail is returned to the sender;
# either the destination rejected the message, or the destination
# could not be reached before the message expired in the queue.
#
А где найти этоту instructions in the bounce(5) manual page.