if Column DataGridViewTextBoxColumn13 has email address and you want to send mail on click event of cell contents of that particular column [All cells of column DataGridViewTextBoxColumn13]. Rewrite the code as -
Private Sub DataGridView1_CellContentClick(sender As Object, e As DataGridViewCellEventArgs) Handles DataGridView1.CellContentClick
Dim ColIndex as Integer =
message.From = New MailAddress(EmailTextBox1.Text)
message.To.Add(DataGridViewTextBoxColumn13.Rows(e.RowIndex).Cell(ColIndex).Value.ToString)
message.Priority = MailPriority.Normal
smtp clients settings
smtp.EnableSsl = True
smtp.Port = 587
smtp.Host ="smtp.office365.com"
smtp.Credentials = New Net.NetworkCredential(EmailTextBox1.Text, PasswordTextBox.Text)
smtp.Send(message)
MsgBox("message Is sent")
End Sub