Sending an email from a trigger with an addError

I am pretty sure that the only way that you can do what you are looking for would be to create a 'Email Services' class rather than using the generic email2case feature - this is a class that implements Messaging.InboundEmailHandler and it requires that you do all of the work yourself (such as ding the message, looking up contacts, determining if the message is a reply to an existing email, etc), but because it runs at a 'lower level' than the email2case, there is much more that you can do.

See the following for details:

https://help.salesforce.com/htviewhelpdoc?id=code_email_services.htm&siteLang=en_US

https://help.salesforce.com/apex/HTViewHelpDoc?id=code_email_services_editing.htm&language=en_US