<appender name="ALARM-APPENDER" class="ch.qos.logback.classic.net.SMTPAppender">
...
<cyclicBufferTracker class="ch.qos.logback.core.spi.CyclicBufferTracker">
<maxNumberOfBuffers>256</maxNumberOfBuffers>
</cyclicBufferTracker>
...
</appender>

Configured like above, bug got error messages:
10:00:48,171 |-ERROR in ch.qos.logback.core.joran.spi.Interpreter@82:24 - no applicable action for [maxNumberOfBuffers], current ElementPath is [[configuration][appender][cyclicBufferTracker][maxNumberOfBuffers]]

After checking the source code, I know CyclicBufferTracker has refactored from a interface to a Class.

But after "setMaxNumberOfBuffers" method was removed:
https://github.com/qos-ch/logback/commit/773193a09922ab2a1dd70e4978be8cdf4e050f6e
There is no place to config "maxNumberOfBuffers" attribute.

Actually, the only "maxNumberOfBuffers" String can be found in github repository, is in the logback user manual:
https://github.com/qos-ch/logback/search?q=maxNumberOfBuffers
http://logback.qos.ch/manual/appenders.html#bufferManagement
http://logback.qos.ch/recipes/emailPerTransaction.html