SmtpClient.BeginTestRecipients Method

Tests email address asynchronously.

[Visual Basic]
Public Function BeginTestRecipients( _
    server As SmtpServer, _
    mail As SmtpMail, _
    callback As AsyncCallback, _
    state As Object _
) As SmtpClientAsyncResult

[C#]
public SmtpClientAsyncResult BeginTestRecipients(
    SmtpServer server,
    SmtpMail mail,
    AsyncCallback callback,
    Object state
);

[C++]
public: SmtpClientAsyncResult^ BeginTestRecipients(
    SmtpServer^ server,
    SmtpMail^ mail,
    AsyncCallback^ callback,
    Object^ state
);

[JScript]
public function BeginTestRecipients( 
    server : SmtpServer, 
    mail : SmtpMail, 
    callback : AsyncCallback, 
    state As Object 
) : SmtpClientAsyncResult;

Parameters

server: A SmtpServer instances used to test recipients.
mail: A SmtpMail instance to test.
callback: The AsyncCallback delegate.
state: An object containing state information for this request.

Return Value

An SmtpClientAsyncResult that references the asynchronous send.

Remarks

How does it work? Firstly, SmtpClient performs a DNS MX record query. If it retrieves the recipient's local SMTP server successfully, SmtpClient will try to connect to this server. SmtpClient then performs "RCPT TO" command to test if this SMTP server accept this email address.

Please always pass null (Nothing in Visual Basic) to SmtpServer paramter except you want to test whether an email address will be accepted by a specified SMTP server.

Example

[Visual Basic, C#, C++] To get the full samples of EASendMail, please refer to Samples section.

[VB - Test Email Address]

Imports EASendMail

Sub TestEmailAddress()

Try
    Dim oMail As SmtpMail = New SmtpMail("TryIt")
    oMail.From = New MailAddress("from@adminsystem.com")
    oMail.To.Add(New MailAddress("to@adminsystem.com"))

    Dim oSmtp As SmtpClient = New SmtpClient
    Dim asyncResult As SmtpClientAsyncResult = oSmtp.BeginTestRecipients(Nothing, oMail, Nothing, Nothing)
    Do While Not asyncResult.AsyncWaitHandle.WaitOne(5, False)
        ' do other thing
    Loop

    oSmtp.EndTestRecipients(asyncResult)
    Console.WriteLine("PASS")

Catch exp As Exception
    Console.WriteLine("Exception: {0}", exp.Message)
End Try

End Sub


[C# - Test Email Address]

using System;
using EASendMail;

void TestEmailAddress()
{
    try
    {
        SmtpMail oMail = new SmtpMail("TryIt");
        oMail.From = new MailAddress("from@adminsystem.com");
        oMail.To.Add(new MailAddress("to@adminsystem.com"));

        SmtpClient oSmtp = new SmtpClient();
        SmtpClientAsyncResult asyncResult = oSmtp.BeginTestRecipients(null, oMail, null, null);
        while (!asyncResult.AsyncWaitHandle.WaitOne(5, false))
        {
            // do other thing
        }

        oSmtp.EndTestRecipients(asyncResult);
        Console.WriteLine("PASS");
    }
    catch (Exception exp)
    {
        Console.WriteLine("Exception: {0}", exp.Message);
    }
}
        

[C++/CLI - Test Email Address]

using namespace System;
using namespace EASendMail;

void TestEmailAddress()
{
    try
    {
        SmtpMail ^oMail = gcnew SmtpMail("TryIt");
        oMail->From = gcnew MailAddress("from@adminsystem.com");
        oMail->To->Add(gcnew MailAddress("to@adminsystem.com"));

        SmtpClient ^oSmtp = gcnew SmtpClient();
        SmtpClientAsyncResult ^ar = oSmtp->BeginTestRecipients(nullptr, oMail, nullptr, nullptr);
        while (!ar->AsyncWaitHandle->WaitOne(5, false))
        {
            // do other thing;
        }

        oSmtp->EndTestRecipients(ar);
        Console::WriteLine("PASS");
    }
    catch (Exception ^exp)
    {
        Console::WriteLine("Exception: {0}", exp->Message);
    }
}

Test Email Address

Example for Testing Email Address
Validate Email Address Syntax and Test Email Address in VB
Validate Email Address Syntax and Test Email Address in C#
How does it work?
Firstly, SmtpClient performs a DNS MX record query. If it retrieves the recipient's local SMTP server successfully, SmtpClient will try to connect to this server. SmtpClient then performs "RCPT TO" command to test if this SMTP server accepts this email address.
Why we don't recommend testing email address?
Because it totally depends on your networking connection, if your networking connection to the recipient server is bad or your IP address is blocked by the recipient server, test will be failed, but it doesn't mean this email address is invalid.

Moreover, to prevent email address testing, many email providers accept the recipient address at first no matter if the address is valid or invalid, only after you sent the email data to the server, then the server rejects it if the recipient address is invalid.
Recommended Solution
Send the email to the recipient without testing. If you don't get Transport Error and Failure Report in 24 hours, that means the recipient is valid. If you get Failure Report, you should consider to remove this recipient from your mail listing.