Everyone is giving good advice here. Using Nmap is legal, however, using Nmap for malicious purposes can get you into trouble if isomeone uses it to break into someone else's system. I use Nmap to merely gather information whenever I have an issue on my local network.

It sounds like you scanned him/her and after the fact realized your activity may be logged like you logged his/her activity, so to cover you tracks or defend what you did, you emailed the admin for his/her domain and attempted to indirectly justify what you did.

The person you scanned may very well report you to your isp as well just as you did him/her. Be careful what you use Nmap for, it can definitely get you in trouble if not used appropriately.