Configure log4net to send errors to different appenders based on level

This can be done using the threshold or filter elements within the appender.

Note that the threshold can be directly under the appender, where it acts as an inclusive filter, or under a evaluator e.g.

<evaluator type="log4net.Core.LevelEvaluator">
  <threshold value="ERROR"/>
</evaluator>

where it acts as an inclusive filter for skipping buffering (immediate output), where applicable.

Full explanation (source):

<threshold value="ERROR" />

The Threshold is implemented in the AppenderSkeleton and therefore
supported by almost all appenders. It is just a simple test that is
used to ignore logging events that have a level below the threshold.
The threshold is checked early and as a simple test is very
performant.

There is another way to specify the same behaviour as the threshold
using filters. Filters are very much more flexible and as they are
pluggable you can also develop your own custom logic and insert it
into the filter chain.

<filter type="log4net.Filter.LevelRangeFilter">
    <levelMin value="ERROR" />
    <levelMax value="OFF" />
</filter>

Like the Threshold check Filters are implemented in the AppenderSkelton base class and are supported by almost all appenders. The above filter has the same effect as <threshold value="ERROR" />. It is a LevelRangeFilter that will allow through any events with a level in the range ERROR to OFF (inclusive). Note that OFF is the name of the highest level, conversely ALL is the name of the lowest level.

Filters have a great deal of flexibility because multiple filters can
be chained together to provide fine grained control over the events
that are output. Because of this they also have a higher cost in terms
of performance, each filter in the chain is an object and is asked to
decide on the correct course of action. In the simple case of the
threshold filtering the Threshold property should be used in
preference to a filter.

The Evaluator is implemented by the BufferingAppenderSkeleton and
is therefore only supported by appenders that extend this base class
and provide support for buffering. The SmtpAppender is one such
appender.

The Evaluator is a pluggable object that is used by the
BufferingAppenderSkeleton to determine if a logging event should not
be buffered, but instead written/sent immediately. If the Evaluator
decides that the event is important then the whole contents of the
current buffer will be sent along with the event. The evaluator does
not function like the threshold or a filter in that it does not
discard events.