Email address 'user at domain.de' not in local domain

[Henning]

Hi,

I have a kolab setup with one primary domain, assume "domain.com". My
kolab server receives incoming mail for in total three domains
domain.com, domain.de and domain.net.
Mail reaches kolab-users because I configured alias entries for all the
three domains. That worked for a long time. Today I discovered, that
adding another alias via kolab-webadmin is not possible anymore. It
throws the exception

Email address 'user at domain.de' not in local domain.

(error while calling _validate_email_address_in_any_of_my_domains near
line 1116 in kolab_api_service_form_value.php)

mailHost for all users is and was always "localhost" (default, not
changeable in kolab-webadmin). The hostname of my machine is
"mail.domain.com" -- maybe this has something to with this (changed
somewhat recently)?

What's wrong here? Please help me out.

Henning