In VB.Net, I use this small method:

Private Sub SendMail(ByVal pstrSubject As String, ByRef pstrMessage As String)
Dim ms As New MailMessage()

ms.From = "from@from.com"
ms.To = "to@to.com"
ms.Subject = pstrSubject
ms.Body = pstrMessage
ms.BodyFormat = MailFormat.Text
SmtpMail.SmtpServer = "my.server.com"
SmtpMail.Send(ms)
'MsgBox("pstrMessage=" + Len(pstrMessage).ToString + vbCrLf + "ms.Body=" + Len(ms.Body).ToString)
End Sub

My message is about 10,000 characters long and is getting truncated around 3300 characters.

Why?