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Little mistake in GetStandardHeaders.
It must be replace
standardheaders.Add(new MessageHeader("From", _fromaddress.ToDataString()));
standardheaders.Add(new MessageHeader("To", _toaddresses.ToDataString()));
with
standardheaders.Add(new MessageHeader("From", _fromaddress.ToDataString()));
standardheaders.Add(new MessageHeader("To", _toaddresses.ToDataString()));
standardheaders.Add(new MessageHeader("Reply-To", _replytoaddresses.ToDataString()));