Email etiquette

People have recently started sending me Outlook meeting notifications. I assume that these are generated automatically when they put items into their Outlook calendar, and are primarily intended to talk to other Outlook calendars.

I don't use Outlook, so the notifications just come through as rather curt looking autogenerated messages.

My etiquette question is: does one reply to these messages, as if they had been sent by a person, or does one take them as machine-generated, and respond only if one cannot make the scheduled time?