| A DKIM_PSTATE is an opaque structure which contains state
    information with respect to policy lookups. 
    DKIM policy evaluation is a multi-step process which can involve several
    queries to look up SSP or other information about a sending domain.  When
    a policy lookup callback is in use
    (see dkim_set_policy_lookup()),
    the callback might be unable to complete a particular lookup and thus
    return DKIM_CBSTAT_TRYAGAIN.  In such cases, it is likely
    desirable to resume the incomplete policy lookup when the calling
    application knows it can complete the query.  The DKIM_PSTATE
    is a state structure which is loaded with information required to resume
    a policy evaluation where a callback caused it to be suspended.  
    Before use, it should be allocated by the caller and cleared using
    bzero() or memset(). |