First you need to create an initialization module as a container for your override code. You will find details of how to do this here:

http://world.episerver.com/Documentation/Items/Tech-Notes/EPiServer-CMS-6/EPiServer-CMS-60/Initialization/#Creating

Make sure your module is dependent on the EPiServer.Web.InitializationModule using the ModuleDependency attribute:

[ModuleDependency(typeof(EPiServer.Web.InitializationModule))]

In the Initialize method of your module you can add the following code to customize the email output:

BaseLibrary.Channel.IChannel channel = new SynchronousChannel("xformdata");

EPiServer.XForms.Implementation.EMailConnection emailSender = new EPiServer.XForms.Implementation.EMailConnection();

emailSender.Subject = "MySubject"

emailSender.XMailer = "EPiServer XForms";

emailSender.XsltUrl = url to an Xslt file that transforms the XML Document containin the post;
emailSender.From = "";
emailSender.To = "";

channel.SendEvent += new BaseLibrary.Channel.ChannelEventHandler(emailSender.SendEmail);

channel.SendEvent += new BaseLibrary.Channel.ChannelEventHandler(new EPiServer.XForms.Implementation.HttpConnection().HttpRepost);

channel.SendEvent += new BaseLibrary.Channel.ChannelEventHandler(XForms.XFormData.SaveEventHandler);

BaseLibrary.Context.Repository.UnregisterChannel("xformdata");

BaseLibrary.Context.Repository.RegisterChannel(channel);

Please note that I have not compiled or tested this code :-)