I have many different website accounts with your service. Can I log into the same web mail URL to check mail for all of my accounts?

No. Each unique website account will have a unique webmail URL that you can log into.

For example: To access email for myfirstsite.com, you must log in to webmail.myfirstsite.com, and to access email for mysecondsite.net, you must log in to webmail.mysecondsite.net.

Or, you must login to the cPanel with the unique cPanel credentials for each domain.