主题：测试发送邮件失败

 应该是用到了 SSL 安全，试试

Dim client As New System.Net.Mail.SmtpClient("smtp.qq.com", 25)
client.EnableSsl = True 'SSL
client.Credentials = New System.Net.NetworkCredential("455189993@qq.com", "helen6849338.")
client.DeliveryMethod = System.Net.Mail.SmtpDeliveryMethod.Network
Dim message As New System.Net.Mail.MailMessage()

message.From = new System.Net.Mail.MailAddress("455189993@qq.com","别名")

message.To.Add(new System.Net.Mail.MailAddress("2450314695@qq.com","人名"))
message.To.Add("2450314695@qq.com")
message.Subject = "邮件测试"
message.BodyEncoding = System.Text.Encoding.UTF8

Dim content As String

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

message.Body = content

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