With LDAPAuthentication one can get the mail attribute filled from LDAP as defined in config/modules/authentication-ldap.cfg with email_field set.
But the following happens with DSpace 4.x: the email address received by
the LDAP server is always overwritten with the netid.
It looks like this behavior was introduced with the following commit in order to fix
(lines 261 to 264 ).
The attached patch would fix this behaviour (with courtesy to rlseaton: http://dspace.2283337.n4.nabble.com/DSpace-LDAP-authentication-problem-td4665853.html#a4668861 )