And we don't even know if we can trust its "mail" attribute... The same problem is with LDAP. We can't assume mail-server is installed, too!
We have a similar problem with password expiration notifications: what is the user's mail address? Only the user knows!
In other words, as said, if dokuwiki has an "email" field in its user profile form, I'd expect the user sets it to his preferred email address by hand.