Horde can not expand email addresses

I have an Addressbook with values in the "Alias" field for each entry. When composing a msg in Horde, if I click the "Addresses" link I can view and select any entry in my address book; that entry is written correctly into the address box (eg, the 'to' box). However, if I simply enter a valid alias in, for example, the "to" box, I receive this error msg: "The field "to" can't be expanded."; the alias is not replaced with the recipient's name and email address.
In my Mail\Options for Mail\Address Books configuration settings, under "Select all fields to search when expanding addresses" only the Name and Email fields are listed. Both are selected/enabled.

Q: Have I misunderstood the concept of 'expanding'?
Q: Should my "Select all fields..." list include the "Alias" field?
(If so, how do I add it?)
Any tips on how to resolve this problem will be appreciated.