Ignore configured domains in attr_mail

We have the situation that mails for members of our organisation are managed on host A, and mails for guests are managed on host B. Members are on the teckids.org domain, guests have some domains to choose from. They are fed from the same LDAP, and a person can have both a member address and a guest address (for fun, et al).

For delivery, all addresses are kept in the mailLocalAddress attribute. We do not want the member identities to be available in the guest roundcube, so we ignore the organisational domain parts there.

Others could find that useful too, and actually it is fully compatible to what we had before.