This property has nothing to do with your emails being delivered or not.
As the documentation/intelisense states:
ClientBase.ReceiveTimeout gets or sets the amount of time the underlying Socket will wait to receive data once a read operation is initiated.
Are you checking the return value of Smtp.SendMessage method?
using (Smtp smtp = new Smtp())
{
smtp.Connect("server.example.com"); // or ConnectSSL for SSL
smtp.UseBestLogin("user", "password"); // remove if authentication is not needed
ISendMessageResult result = smtp.SendMessage(email);
if (result.Status == SendMessageStatus.Success)
{
// Message was sent.
}
else
{
string asString = result.ToString();
// You can use other ISendMessageResult properties here:
List<string> rejected = result.RejectedRecipients();
}
smtp.Close();
}