[Exim] over-quota delivery failure text

I'm probably being stoopid but I've looked in the FM but I can't find how to set the
over-quota delivery failure text

to anything other than

  juser@???
             mailbox is full (MTA-imposed quota exceeded while writing to file
             /exposed/path/to/mailbox):

Can anyone help - or point me to the right place in the manual

TIA




--
Jonathan Kyme
jrk@???