Can't set altRecipient; what am I doing wrong

Hi,

I'm trying to set the altRecipient property (usede for email forwarding) in a user object without success. Here's the code:

DirectoryEntry entry = new DirectoryEntry("LDAP://" + userDN);
entry.Properties["altRecipient"].Value = forwardDN;
entry.CommitChanges();
entry.Close();

The error I get is: A constraint violation occurred

I really have no idea about what constraint I'm violating. Can someone please enlighten me.

Thanks,
Rob