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:
LogFormat "%{%F %T}t.%{msec_frac}t %v %{X-Forwarded-For}i %u %{MELLON_urn:mace:dir:attribute-def:mail}e \"%r\" %>s %O \"%{Referer}i\"\ \"%{User-Agent}i\"" vhost_federated_combined
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