This was not a DNS error.
I found the answer when examining IIS where i noticed the website was stopped.
Forcing the website to start, this error message poped up:
IIS was unable to start the site, another site may already be using the port you configured for this site.
Further investigation revealed that one same domain alias has previously added to another domain/host.
Removing this alias from IIS > Website Properties > Website > Advanced > Advanced Website Identification, fixed the problem.
What led me to assume a dns problem mistakenly was that default IP of server is set to mail server by default. so, when a website is stopped the domain points to mail server.
Hope this help for future.