Make sure you can debug all incoming attributes: https://github.com/UNINETT/mod_auth_mellon/blob/master/doc/user_guide/mellon_user_guide.adoc#94-inspecting-mellon-environment-variables. There should be many environment vars, and you need to pick one that is prefixed with MELLON_. Probably MELLON_urn:mace:dir:attribute-def:mail will be the correct one.
Define a custom LogFormat for apache, where you add this environment variable. An example: