We recently had a problem installing Courier (2.7) into an umbraco site that was already using log4net 2.0.0.0 in other parts of the project.

[FileLoadException: Could not load file or assembly
'log4net, Version=1.2.10.0, Culture=neutral, PublicKeyToken=1b44e1d426115821'
or one of its dependencies. The located assembly's manifest definition does not
match the assembly reference. (Exception from HRESULT: 0x80131040)]

It turns out the solution is quite straightforward, to run both versions of log4net side by side.

Copy the log4net 1.2.10.0 dll to a directory in your web project (you don't need to add a reference to it), e.g.

wwwroot\log4net-1.2\log4net.dll

Now add to your web.config:

<configuration>
  <runtime>
    <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
        <assemblyIdentity name="log4net" publicKeyToken="1b44e1d426115821" />
        <codeBase version="1.2.10.0" href="log4net-1.2\log4net.dll" />
      </dependentAssembly>
    </assemblyBinding>
  </runtime>
</configuration

Then install Courier, and everything should work correctly.  Hope this helps someone else.

Credit to Joel Gauvreau on Stackoverflow for the solution:
http://stackoverflow.com/questions/3158928/referencing-2-differents-versions-of-log4net-in-the-same-solution