When binding as a user, one can remove the mail field but lacks write access to add it back. Petteri said that the mail field is not used for anything (atm) so this is minor.
In case it's actually used for something I put darkside@gentoo.org back there.
It's used in retirement processing. Can you give me a testcase that shows how you were doing it? The LDAP ACLs should allow you to write and add it fine.
Sorry for my delay. Either PEBKAC or something changed in the meantime. No issues anymore.