Log4net is de-facto standard in .NET logging, however we can't force users to use it.
This is why Mail.dll uses "soft" reference to log4net - if the library is not found, it is not used, nor required.
This is a common practice while referencing logging tools.
You can avoid it by adding a reference to log4net.dll, but as I explained this is not necessary, you can safely ignore this warning.