It may have something to do with the email address field being an array. Here is link to another UDF that never got published to the UDFs...

GetEmailAddresses()
http://www.kixtart.org/forums/ubbthreads.php?ubb=showflat&Number=188397#Post188397

The sample code is in the posted link and down below are the updated versions of the functions.

If I can find some time, I look it over again, and post at least the GetEmailAddresses to the UDFS. The Set version required modifying permsisions on the Exchange server and I have no desire to get into what it required to change them.