...параметр throwException передать как true.
bool err = true;
Tuple<string, string, string, string>[] allMails;
allMails = ZennoPoster.
BulkMailDownload("
[email protected]", "ххххххх", "imap.gmail.com", 993, true, ZennoLab.InterfacesLibrary.Enums.Email.EmailProtocol.IMAP, 24*100, 20, err );