Rule reminder
In formal correspondence, prefer 'a meno che (non)', 'salvo che (non)', and 'a condizione che' + subjunctive. Avoid informal shortcuts. Fixed phrases like 'salvo imprevisti' (barring unforeseen events) and 'salvo diversa indicazione' (unless otherwise indicated) are very common.
Which is the most formal way to write 'unless you confirm by Friday'?