Assuming you have "Allow log in via email" already enabled, it should work as long as the user already exists in Moodle and she has the email address already filled in. In other words, either the user has already logged in at least once using their username (and synced her email address from LDAP), or you have run the LDAP sync task and created the user (and synced her email address).
If the user doesn't already exist in Moodle and has the email address already filled in, Moodle can't link the email address to the username and can't perform the authentication successfully (the username field is what's really needed to perform the authentication)
I just setup Moodle with OpenLDAP to test it, and as long as you fullfil the above condition, it works as expected
Saludos.
Iñaki.