This forwarding is done "behind the scenes," so when you receive the email the "to:" field will still show the email alias, based on easydns-example.com, rather than the address forwarded to, in this example based on isp.com. Someone looking through the "headers" of the message (a technical list of the servers the email passed through and the times involved) can see that it was forwarded to another address, but at a glance the email looks like it was simply sent to the address associated with your domain.

Replying can be a slightly more complicated issue, as the reply-to address in your message would usually be set to the actual account you are using, and our mailservers cannot affect what shows up there. Thus, if someone emails you at the easydns-example.com address and you reply to it, they will see the isp.com address in the reply-to field of the message they receive.