By default, the builtin templates are used 
The file that you're searching is stored in a dedicated package. In order you must:
1. Install the postfix-doc distribution package: # apt-get install postfix-doc
2. Create the custom bounce template as stated in BOUNCE(5):
3. Translate the template that is now located at /etc/postfix/custom_bounce_tpl.cf
4. Make postfix aware of that custom bounce template by adding the following parameter in the /etc/postfix/main.cf file:
5. Restart postfix: service postfix restart
See also: http://www.postfix.org/bounce.5.html (BOUNCE(5))
/etc/postfix/custom_bounce_tpl.cf
I think it must be /etc/imscp/postfix/custom_bounce_tpl.cf
Is it any way to avoid to be deleted by an imscp update?
Thank you very much
I think it must be /etc/imscp/postfix/custom_bounce_tpl.cf
You must create it in the /etc/postfix directory. I made a mistake in my previous post which is now fixed.
For the parameter to add in the /etc/postfix/main.cf file, you can make it persistent using a i-MSCP listener file.