System.Net.Mail.SmtpStatusCode
BadCommandSequence = 503
ClientNotPermitted = 454
CommandNotImplemented = 502
CommandUnrecognized = 500
GeneralFailure = -1
HelpMessage = 214
InsufficientStorage = 452
LocalErrorInProcessing = 451
MailboxBusy = 450
MailboxNameNotAllowed = 553
MailboxUnavailable = 550
MustIssueStartTlsFirst = 530
Ok = 250
ServiceNotAvailable = 421
ServiceReady = 220
StartMailInput = 354
SyntaxError = 501
SystemStatus = 211
TransactionFailed = 554
UserNotLocalWillForward = 251