I apologise for the delay in my reply.
'Scuso' = SCOO-zo; the 's' is unvoiced before 'c'.
Use whenever you are replying later than expected or have let an email go unanswered for more than two or three days.
'Mi scuso' (I apologise) is formal. In everyday speech Italians often say 'mi dispiace' (I'm sorry), but in writing 'mi scuso' sounds more professional and accountable. Keep the apology brief — a single sentence is enough before moving to the substance.
La prego di scusare il ritardo.
I beg you to excuse the delay.
Very formal; 'La prego' = I beg/request you
Scusa se rispondo solo ora,
Sorry for only replying now,
Informal tu form for colleagues
Mi sono assentato/a per alcuni giorni, ecco perché rispondo tardi.
I was away for a few days, which is why I am replying late.
Adds brief explanation; appropriate if you were truly absent
Italians generally understand that delays happen, especially around August (Ferragosto) when most of the country shuts down for two to four weeks. Always set an out-of-office auto-reply during Italian holidays.