Over quota, mail bounce back: customise error message

Hi everyone,

If a user exceeded his/her quota, mail bounce back to sender with the message "can't create user output file", which is correct from a technical perspective (the server can not write into the user's directory). However, the error message is meaningless for the sender and we have received several support requests from users over time, who assume, there is a technical issue on the server when people informed them about this error message.

I understand, this is not directly a virtualmin issue, but postfix (I assume?). I wonder if it is possible to customise/change this error message in bounce back mails, if the cause of the issue is the quota limit?

Cheers Michael