Jan,

please follow-up on the mailing list (Cc:-ed) so others can answer and learn as well.

Jan Beerden wrote:

We already tried that. We are trying to prevent user1 to have an email alias that is the same as user2's primary email address. [..] What we would like is that a certain value (email address) has to be unique across both the email and the alias attribute.

Uniqueness is only per attribute. So this is not possible.

Ciao, Michael.