| NOTES | 
DKIM_CBSTAT_CONTINUE should be returned by the callback
    on normal successful completion, indicating to library that normal
    operation should proceed
DKIM_CBSTAT_REJECT should be returned by the callback if
    the callback is able to determine that the message should be rejected;
    this will cause the library function which used the callback to
    return DKIM_STAT_CBREJECT 
DKIM_CBSTAT_TRYAGAIN should be returned by the callback if
    the callback is unable to process the message at this time but a later
    attempt might succeed; this causes the library function which used
    the callback to return DKIM_STAT_CBTRYAGAIN, and the caller can
    re-attempt that call later
DKIM_CBSTAT_NOTFOUND should be returned by the callback if
    the record being sought was not found
DKIM_CBSTAT_ERROR should be returned by the callback if
    the attempt to retrieve the requested record returned in some kind
    of error
DKIM_CBSTAT_DEFAULT should be returned by the callback if it does
    not wish to return a result and instead wants the library to proceed
    as if the callback were not configured
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