Loading the assembly for a custom log4net appender

As I described recently, I have built a custom appender that inherits from SmtpAppender but supports SSL sending using .NET 2.0.

However, when I configure my web application to use it, I get an error that:
log4net:ERROR XmlHierarchyConfigurator: Could not create Appender [EmailAppender] of type [log4netExtensions.SmtpClientAsyncSmtpAppender]. Reported error follows.
System.TypeLoadException: Could not load type [log4netExtensions.SmtpClientAsyncSmtpAppender]. Tried assembly [log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=1b44e1d426115821] and all loaded assemblies

It can't load the new appender since that assembly isn't loaded.  To specify that an appender comes from a non-log4net assembly, you add the assembly name after the appender.  For example,

<appender name="EmailAppender" type="log4netExtensions.SmtpClientAsyncSmtpAppender, log4netExtensions">

Note that you need to ensure that assembly is available, so you may need to add a reference to it in your project.