How do I update the "to" addresses on an SMTPAppender in Logback?

I am creating an admin page which has a couple of logback properties that I want to set on the fly, one of them being the admin emails that I send system alerts to. The api for the SMTPAppender has the methods to add to the list of "to" addresses, or get them as a list, but I didn't find anything to clear, remove any or update them. How should I do this?

I see two options currently:

1. One option is to remove the appender and create a new one with the new properties (yuck).
2. Figure out how to configure this directly through Joran (maybe yuck?).

I'm moving forward with (2), but please post if there's a better way.

Maybe you're done with this, but I just needed to find a way to set dynamic "to" addresses, and this topic lead me to the way (thanks to @gresdiplitude idea on system properties and MDC values), so I'm sharing my solution.

I am using SMTPAppender to send small execution reports, but those need to be sent to different mailboxes. The solution looks yuck-less, or at least I got very pleased with the result.

My logback.xml:

<!-- this is the trick: a converter to use on the "to" field pattern -->
<conversionRule conversionWord="smtpTo" converterClass="com.example.logback.MyConverter" />
<appender name="SMTP" class="ch.qos.logback.classic.net.SMTPAppender">
    <!-- a filter to select just the log entries that will be sent by mail -->
    <filter class="com.example.logback.MyFilter" />
    <!-- an evaluator, mine is like CounterBasedEvaluator from the manual:
        http://logback.qos.ch/xref/chapters/appenders/mail/CounterBasedEvaluator.html
    -->
    <evaluator class="com.example.logback.MyEvaluator">
        <limit>25</limit>
    </evaluator>
    <!-- a discriminator to create a cyclic buffer for each mailing group I need -->
    <discriminator class="com.example.logback.MyDiscriminator" />
    <!-- just matching buffer size to evaluator limit -->
    <cyclicBufferTracker class="ch.qos.logback.core.spi.CyclicBufferTrackerImpl">
        <bufferSize>25</bufferSize>
    </cyclicBufferTracker>
    <smtpHost>${smtp.host}</smtpHost>
    <smtpPort>${smtp.port}</smtpPort>
    <SSL>${smtp.ssl}</SSL>
    <username>${smtp.username}</username>
    <password>${smtp.password}</password>
    <from>${smtp.from}</from>

    <!-- here you use the converter: in this case will get data
        from marker containing the destination addresses
    -->
    <to>%smtpTo</to>
    <subject>my subject</subject>
    <layout class="ch.qos.logback.classic.PatternLayout">
        <pattern>%date: %message%n%xThrowable{full}</pattern>
    </layout>
</appender>

MyFilter.java:

public FilterReply decide(ILoggingEvent event) {
    return event.getMarker() != null
            && event.getMarker().contains("REPORT") ? FilterReply.ACCEPT
            : FilterReply.DENY;
}

MyDiscriminator.java:

public String getDiscriminatingValue(ILoggingEvent e) {
    Marker marker = e.getMarker();
    if (marker == null || !(marker instanceof MyMarker)) {
        return null;
    }
    return ((MyMarker) marker).getDiscriminatingValue();
}

MyConverter.java:

public class MyConverter extends ClassicConverter {

    @Override
    public String convert(ILoggingEvent event) {
        Marker marker = event.getMarker();
        if (marker == null || !(marker instanceof MyMarker)) {
            return null;
        }
        return ((MyMarker) marker).getSmtpTo();
    }
}

MyMarker.java:

public interface MyMarker extends Marker {
    // a list of destination addresses, like "a@c.com, x@y.net"
    String getSmtpTo();
    // an "id" to tell the buffers apart, could be "smtpTo" itself
    // but in my case it would mix different reports that goes to the same addresses
    String getDiscriminatingValue();
}

I just created an implementation for MyMarker, and used several instances of it on every logging statement that should be reported:

// suggestion: make the marker immutable, then you can store and reuse them instead of recreating them every time
Marker marker1 = new MyMarkerImpl(
    "REPORT", // marker name
    "me@company.com, joe@company.com", // smtpTo
    "alertGroup"); // discriminatingValue
logger.warn(marker1, "SNAFU");
Marker marker2 = new MyMarkerImpl(
    "REPORT", "boss@company.com, ceo@company.com", "phbGroup");
logger.info(marker2, "Everything is fine");
// here we have same smtpTo as above but different discriminatingValues, so this will be sent in another email/report
Marker marker3 = new MyMarkerImpl(
    "REPORT", "me@company.com, joe@company.com", "bugFixingGroup");
logger.error(marker3, "Why things gone bad", exception);

Hope it can be useful.