OK, I know I could put:

bounce_template_file = /etc/postfix/bounce.cf

in main.cf but it would get overwritten on a reboot/restart.

So, how/what's best way to change the message:

This is the Postfix program at host smtp.mydomain.com

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 Postfix program

<xxx.xxxxaol.com>: host mailin-02.mx.aol.com[] said: 550
NOT FOUND (in reply to RCPT TO command)
Didn't see as one of the options for: zmprov gacf or just zmprov .
Is there a master file somewhere?

Or did I just search through the forums wrong...... ;-)

I would like it to be:

For further assistance, please send mail to myemailaddress@mydomain.com