In whitelist properties, host names are fully-qualified. Thus stives is not a valid host name while stives.customers.mydomain.com is valid. A host name expression of .*customers.mydomain.com would match all of these hosts:
stives.customers.mydomain.com
cour.customers.mydomain.com
tempcustomers.mydomain.com
Note that an expression of .*.customers.mydomain.com would also match these same three hosts.
You may need to specify literal dot separators in host names also to properly clarify the expressions. If in this example you did not want tempcustomers.mydomain.com to be matched, you would need an expression such as .*\.customers.mydomain.com. See Specifying Literal Dot Separators for more information.