For some tasks pre-send scripts are not sufficient, like rewriting the
Message-ID based on the SMTP response for later In-Reply-To headers.
This is needed when using Amazon Simple Email Service (AWS SES) for
sending emails.

Other possible use cases are scripts that act upon failed sending
attempts.

Here is a example post-send script that does the message id rewriting
for AWS SES:

```groovy
import com.sun.mail.smtp.SMTPTransport;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import hudson.plugins.emailext.ExtendedEmailPublisherDescriptor;

String smtpHost = props.getProperty("mail.smtp.host", "");
String awsRegex = "^email-smtp\\.([a-z0-9-]+)\\.amazonaws\\.com\$";
Pattern p = Pattern.compile(awsRegex);
Matcher m = p.matcher(smtpHost);
if (m.matches()) {
    String region = m.group(1);
    if (transport instanceof SMTPTransport) {
        String response = ((SMTPTransport)transport).getLastServerResponse();
        String[] parts = response.trim().split(" +");
        if (parts.length == 3 && parts[0].equals("250") && parts[1].equals("Ok")) {
            String MessageID = "<" + parts[2] + "@" + region + ".amazonses.com>";
            msg.setHeader("Message-ID", MessageID);
        }
    }
}
```

References:
- https://issues.jenkins-ci.org/browse/JENKINS-30854
- #108
- http://docs.aws.amazon.com/ses/latest/DeveloperGuide/smtp-response-codes.html