To access another users mailbox with Exchange WebServices, the primary email address of that mailbox is required. Secondary email addresses just don’t work. But if that all one’s got, the secondary email address needs to be resolved to the primary email address. The ResolveNames operation seems a good candidate for that job. But feeding that operation with a secondary email address will yield no results.
The ResolveNames operation executes an ANR search (Ambiguous name resolution) request on Active Directory, which translates (at least in part) to: (|(…)(proxyAddresses=address*)(…)). But the syntax of the proxyAddresses field is different: It is a multi-valued field where each entry has the form protocol:address. For normal email addresses, an entry looks like this: smtp:email@example.com.
To resolve a secondary email address, the ResolveNames operation has to be called with something like smtp:firstname.lastname@example.org.