Added case clause for successful email delivery.

If the incoming endpoint (read "fake" email address) function doesn't terminate with the atom ok. then the server will resend the response until the server is being shut off.

As of now, the only valid responses are:

ok.
and
{error, Reason}.

By default, the function returns {ok, Pid} which leads to the aforementioned reason because there is no case clause that handles it.