The SMTPAppender class is a part of the Logback library that can be used to send log events as email messages via SMTP (Simple Mail Transfer Protocol). The log events are sent as email when they meet a certain threshold level or when the appender is triggered by the triggering policy you have configured.

Here is an example of how you might use the SMTPAppender class in a Logback configuration file:

<configuration>
  <appender name="EMAIL" class="ch.qos.logback.classic.net.SMTPAppender">
    <smtpHost>smtp.gmail.com</smtpHost>
    <smtpPort>587</smtpPort>
    <STARTTLS>true</STARTTLS>
    <username>[email protected]</username>
    <password>your_password</password>
    <to>[email protected]</to>
    <from>[email protected]</from>
    <subject>Application Error</subject>
    <layout class="ch.qos.logback.classic.html.HTMLLayout" />
  </appender>
  <root level="error">
    <appender-ref ref="EMAIL" />
  </root>
</configuration>

In this configuration file, an appender named "EMAIL" is set up that sends log events that are at the ERROR level and above via email. The element specifies the host of the SMTP server to use, specifies the port for the SMTP server and is used for secure connection and set to true in this case.

It uses the provided gmail email address, a valid email address, and password. It also set the "to" and "from" email address in the appender and the subject of the email.

The layout element defines the format of the log events in the email. In this case, it is using the HTMLLayout, which formats the log events as HTML.

It's important to notice that you need to allow less secure app in case you use a gmail account as the email sender.

You can check logback documentation for more information and options available.