I configured an email service in a sandbox environment. I set it up to receive from all email addresses and to bounce on any sort of error. I also set the service up to route all error emails to my email address. I then added a single email address for this new email service.

In my testing, I could see clearly in the debug logs that my emails were being sent to this service address as expected, with all the calls to Messaging methods looking good. I could see that my service handler class was being considered, because I would receive an email when the class had errors which are caught by static analysis (invalid type casts, in this case). So, I fixed those errors and tried to send to this address again.

Strangely, I cannot at all get my email service handler code to execute in this sandbox. A similar service (the configuration is identical as far as I can see) has run smoothly in our production org for many months and I do not have an immediate reason to suspect it. I added code to this handler which immediately should send me an email with the contents of the inbound message as this notification's body. However, even this simple action never occurs, and I never receive the email from the service handler.

It seems that this is some configuration issue, but allow me to show my code.

Here is the code that I use from the Developer Console to send a simple message to my email service:

Messaging.SingleEmailMessage msg = new Messaging.SingleEmailMessage();
msg.setToAddresses(new String[] {
    /* The email service's address */
});
msg.setSubject('test');
msg.setPlainTextBody('Test');
Messaging.sendEmail(new Messaging.SingleEmailMessage[] { msg });

And here is the code from the top of my service handler:

global Messaging.InboundEmailResult handleInboundEmail(
            Messaging.InboundEmail email, 
            Messaging.InboundEnvelope envelope) {
    String body = email.plainTextBody;
    Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
    String[] testAddresses = new String[] {
        /* My personal address */
    };
    mail.setToAddresses(testAddresses);
    mail.setSubject('Response');
    mail.setPlainTextBody(body);
    Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });

Does anybody have an idea about what is going on? I also tried composing a plain email from my mail client and sending it to the service address, and got no reply/bounce/anything of the sort.