Having a cup of coffee everyday is not harmful and actually coffee can benefit your heart health and other overall health as well.
I drink as much as 8 cups of coffee per day sometimes and I've never experienced any health problems as a result although the coffee does make me pee soon after drinking the coffee since it's a diuretic.
You might also get diarrhea from too much coffee but coffee is healthy in moderation and one cup of coffee per day is certainly in moderation and not bad for you.