When an email is sent to dsaadsadsad@domain.com , and that account does not exist an error eamil is sent back to the sender.
Is there a file that I can edit I can change the content of this return error mesage?

Under the Preferences Option of that domain you'll see the option to either return the e-mail if the address does not match one setup or to use one e-mail address as a catch all on that domain.

that is not what i asked.
I asked how I can customize the error message that is sent to the sender if the email address they sent to does not exist.

The option to return the e-mail to sender is a box with a line of text in, this is what you want to customise.

The default message is:



From where I described in my previous post you can change this message.