for some reason this subdomain for some of my customer redirects to one of my own websites:

webmail.customerdomain.com redirects to my own website, if the customer have not done that subdomain himself before. Why it could be like this and how could I change it?

could I make webmail.customerdomain.com to be default pointing to my webmail service?

Just manually modify the httpd.conf to reflect the path change. Of course make backups of the httpd.conf BEFORE you start!!!