ASP.NET VB调用SMTP发邮件报System.Net.Mail.SmtpException错误如何解决

错误说明 #

你遇到的System.Net.Mail.SmtpException: Failure sending mail错误，根因是内部SSL握手失败：System.IO.IOException: The handshake failed due to an unexpected packet format，该错误由SMTP配置不匹配直接导致。

现存配置问题 #

你的代码存在三处可直接触发报错的问题：

1. 凭据配置冲突：同时开启了默认凭据和自定义凭据，UseDefaultCredentials = True会覆盖后续设置的账号密码，导致SMTP认证失败
2. 端口与SSL配置不匹配：不同SMTP端口对SSL的支持规则不同，混用会直接导致握手失败
3. 代码语法错误：Catch异常输出的代码末尾多了一个多余的双引号，会引发编译错误

修复步骤 #

• 调整凭据配置顺序，禁用默认凭据：将smtp_server.UseDefaultCredentials = True改为smtp_server.UseDefaultCredentials = False，且必须放在smtp_server.Credentials赋值语句之前
• 匹配端口与SSL规则：根据你的SMTP服务商配置二选一测试：
  • 若服务商支持587端口STARTTLS：保留smtp_server.Port = 587和smtp_server.EnableSsl = True
  • 若服务商仅支持465隐式SSL：注意旧版.NET Framework的SmtpClient原生不支持465端口，可替换为端口25+EnableSsl=False临时测试，生产环境建议升级框架或使用第三方邮件发送库
• 修正语法错误：将报错行的多余双引号删除，改为Response.Write("Failed to deliver message to " & i & " error::: " & ex.InnerExceptions(i).FailedRecipient)

修正后参考代码 #

<%@ Page Language="VB" Debug="true" %>
<%@ Import Namespace="System.Net.Mail" %>
<script runat="server"> 
    Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
        Dim Subject As String = "This is test mail with html web page"
        Dim ToEmail As String = "toemail@sample.com"
        Dim SMTPUser As String = "info@mysite.com"
        Dim SMTPPassword As String = "mysamplepass"
        Dim e_mail As New System.Net.Mail.MailMessage()
        e_mail.From = New MailAddress(SMTPUser)
        e_mail.To.Add(ToEmail)
        e_mail.Subject = "testEmail"
        e_mail.IsBodyHtml = True
        e_mail.Body = "<p>send test message to email</p>"
        e_mail.Priority = Net.Mail.MailPriority.High
        Dim smtp_server As New SmtpClient
        ' 先禁用默认凭据，再设置自定义账号密码
        smtp_server.UseDefaultCredentials = False
        smtp_server.Credentials = New Net.NetworkCredential(SMTPUser, SMTPPassword)
        smtp_server.Port = 587
        smtp_server.EnableSsl = True
        smtp_server.Host = "mail.mysite.com"
        Try
            smtp_server.Send(e_mail)
        Catch ex As SmtpFailedRecipientsException
            For i = 0 To ex.InnerExceptions.Length Step 1
                Dim status As SmtpStatusCode = ex.InnerExceptions(i).StatusCode
                If status = SmtpStatusCode.MailboxBusy Or status = SmtpStatusCode.MailboxUnavailable Then
                    Response.Write("Delivery failed - retrying in 5 seconds.")
                    System.Threading.Thread.Sleep(5000)
                    smtp_server.Send(e_mail)
                Else
                    Response.Write("Failed to deliver message to " & i & " error::: " & ex.InnerExceptions(i).FailedRecipient)
                End If
            Next i
        Catch ex As Exception
            Response.Write("Exception caught in RetryIfBusy(): " & ex.ToString())
        End Try
    End Sub
</script>

内容的提问来源于stack exchange，提问作者Farrokh Seyrafian