Sending an email using SMTP throws a "Mailbox unavailable. The server response was: Too many invalid recipients" Error

Hi there,
i'm trying to send an email with the code below. The thing is that i keep getting this error "Mailbox unavailable. The server response was: Too many invalid recipients" is there something i'm missing?
I'm using VS 2008
Thanks,
COHFL

Public Shared Sub SendNotificationEmail(ByVal emailer As String, ByVal MessageSubject As String, ByVal MessageBody As String)

            'create the mail message
            Dim mail As New Net.Mail.MailMessage()

            'set the addresses
            mail.From = New Net.Mail.MailAddress("username@domain.com")'this email address does exist
            mail.To.Add("username@domain.com") 'this email does exits
            'set the content
            mail.Subject = MessageSubject & " " & Now.ToShortDateString
            mail.Body = MessageBody

            Try
                'set the server to be used
                Dim smtp As New Net.Mail.SmtpClient("mx1.server.com") 
                'send the message
                smtp.Send(mail)
            Catch ex As Exception
                Throw ex.InnerException
            End Try

        End Sub

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