要这样写代码

Dim client As New System.Net.Mail.SmtpClient()
client.Host = "smtp.qq.com"
client.Credentials = New System.Net.NetworkCredential("lin_hailun@qq.com", "helen6849338.")
client.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network
Dim message As New System.Net.Mail.MailMessage("lin_hailun@qq.com", "2450314695@qq.com")
message.Subject = "邮件测试"
message.BodyEncoding = System.Text.Encoding.UTF8

\'message.Bcc.Add("2450314695@qq.com") \'密送

\'message.cc.Add("2450314695@qq.com") \'抄送

Dim content As String

content &= "这是文字" & vbcrlf
content &= "也是文字"

message.Body = content

Dim attachment As new  System.Net.Mail.Attachment("d:\\test.xls",  System.Net.Mime.MediaTypeNames.Application.Octet)
message.Attachments.Add(attachment)

Try
    client.Send(message)
    msgbox("发送成功")
Catch ex As Exception
    msgbox("发送失败:" & ex.message)

finally

    message.Dispose()
End Try

 你是不是设置了【授权码】？要用授权码代替密码

http://blog.csdn.net/youhong8604/article/details/50403512

是用的授权码，不行。