It can be anything! An apex, subdomain, or even a URL.
Results forappriver.com.2.0001.arsmtp.com
www.appriver.com.2.0001.arsmtp.comcom.2.0001.arsmtp.com2.0001.arsmtp.com0001.arsmtp.comarsmtp.comcom
It can be anything! An apex, subdomain, or even a URL.