This can be accomplished by specifying a bounce template file with the bounce_template_file configuration parameter.

To create a customized bounce template file, create a temporary copy of the file /etc/postfix/bounce.cf.default and edit the temporary file.

To preview the results of $name expansions in the template text, use the command
postconf -b temporary_file.

Errors in the template will be reported to the standard error stream and to the syslog daemon.

While previewing  the text, be sure to pay particular attention to the expansion of time value parameters that appear in the delayed mail notification text.

Once  the result is satisfactory, copy the template to the Postfix configuration directory and specify in main.cf something like:

/etc/postfix/main.cf:
bounce_template_file = /etc/postfix/bounce.cf

The template file can specify templates for failed mail, delayed mail, successful delivery or for address verification.  These templates are named failure_template, delay_template, success_template  and   verify_template, respectively. You can but do not have to specify all four templates in a bounce template file.

Each template starts with "template_name = <<EOF" and ends with a line that contains the word "EOF" only. You can change the word EOF, but you can't enclose it in quotes as with the shell or with  Perl (template_name = <<'EOF'). Here is an example:

# This failure template is used for undeliverable mail.

failure_template = <<EOF
Charset: us-ascii
From: no-reply@mail.mydomain.com (Mail Delivery System)
Subject: Undelivered Mail Returned to Sender
Postmaster-Subject: Postmaster Copy: Undelivered Mail

This is the mail system at host $myhostname.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below. For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system
EOF