I recently encountered a perplexing issue with our ASP.NET email sending functionality where the first attempt fails with authentication errors, but subsequent attempts work flawlessly. Here's the exact error message we're seeing:

Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender

This occurs specifically in our test environment running Microsoft Exchange Server. The production environment with identical code works perfectly. Our simplified sending code looks like this:

Dim smtpClient As New SmtpClient("mail.contoso.com")
smtpClient.UseDefaultCredentials = True
mailMessage.From = New MailAddress("sender@contoso.com")
smtpClient.Send(mailMessage)

The most puzzling aspect is the timing behavior:

• First attempt: Always fails with 5.7.1 error
• Second attempt (immediately after): Success
• After period of inactivity: Fails again on first attempt

After extensive testing, we identified this as an Exchange Server authentication caching issue. The key findings:

• The SMTP client initially fails to properly authenticate with Exchange
• The failed attempt somehow "primes" the authentication channel
• Subsequent attempts use cached credentials successfully

Here are three working approaches we've validated:

1. Explicit Credential Specification

Dim smtpClient As New SmtpClient("mail.contoso.com")
smtpClient.Credentials = New NetworkCredential("username", "password", "domain")
mailMessage.From = New MailAddress("sender@contoso.com")
smtpClient.Send(mailMessage)

2. Retry Mechanism

Dim retryCount As Integer = 0
Do While retryCount < 2
    Try
        Dim smtpClient As New SmtpClient("mail.contoso.com")
        smtpClient.UseDefaultCredentials = True
        mailMessage.From = New MailAddress("sender@contoso.com")
        smtpClient.Send(mailMessage)
        Exit Do
    Catch ex As SmtpException When ex.Message.Contains("5.7.1") AndAlso retryCount = 0
        retryCount += 1
    End Try
Loop

3. Pre-authentication

For .NET Framework 4.5+ applications, you can force authentication before sending:

Dim smtpClient As New SmtpClient("mail.contoso.com")
smtpClient.UseDefaultCredentials = True
smtpClient.TargetName = "SMTPSVC/mail.contoso.com" 'SPN format for Exchange
mailMessage.From = New MailAddress("sender@contoso.com")
smtpClient.Send(mailMessage)

If you have access to Exchange Server, verify these settings:

• Receive Connector authentication methods
• Permission groups on the Receive Connector
• AD permission for the sending service account

The variation between environments typically stems from:

• Different Exchange versions or update levels
• Varying authentication protocol configurations
• DNS resolution differences
• Service Principal Name (SPN) registration

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Recently, while working on an ASP.NET web application that sends emails through Exchange Server, I encountered a particularly puzzling behavior. The first email send attempt consistently fails with:

Mailbox unavailable. The server response was: 5.7.1 Client does not have permissions to send as this sender

Yet, if the user immediately retries the operation, it succeeds without any changes to the code or configuration.

After extensive testing and research, I discovered this occurs due to how Exchange handles SMTP authentication when using UseDefaultCredentials = True. The sequence works like this:

1. First attempt: Exchange challenges for authentication
2. Second attempt: Credentials are properly negotiated
3. Subsequent attempts remain authenticated until timeout

The issue stems from the NTLM authentication protocol used by Exchange. NTLM requires multiple round trips between client and server:

• First connection establishes the authentication context
• Second connection completes the authentication
• This explains why the first attempt fails but retries succeed

Here are three working approaches I've validated:

Solution 1: Pre-authenticate Connection

// Create and dispose a test connection first
using (var testClient = new SmtpClient("your.exchange.server"))
{
    testClient.UseDefaultCredentials = true;
    testClient.Send(new MailMessage("from@domain.com", "to@domain.com", "Test", "Body"));
}

// Now the real connection will work
var realClient = new SmtpClient("your.exchange.server");
realClient.UseDefaultCredentials = true;
realClient.Send(yourActualMessage);

Solution 2: Implement Credential Caching

private static NetworkCredential _cachedCredential;

public void SendEmail(MailMessage message)
{
    var client = new SmtpClient("your.exchange.server");
    
    if (_cachedCredential == null)
    {
        client.UseDefaultCredentials = true;
        // First attempt might fail but caches credentials
        try { client.Send(message); }
        catch { /* Ignore first failure */ }
        
        _cachedCredential = client.Credentials as NetworkCredential;
    }
    
    client.Credentials = _cachedCredential;
    client.Send(message); // This will work
}

Solution 3: Switch to Explicit Credentials

var client = new SmtpClient("your.exchange.server")
{
    Credentials = new NetworkCredential("username", "password", "domain"),
    EnableSsl = true
};
client.Send(yourMessage);

The described behavior typically only appears in development/test environments because:

• Production environments often have persistent connections
• Load balancers may maintain authentication state
• Different Exchange configuration settings are applied

If you're still experiencing issues:

1. Verify the service account has "Send As" permissions in Exchange Admin Center
2. Check if the Exchange server requires TLS 1.2 (enable it in your .NET config)
3. Test with a packet analyzer like Wireshark to see the SMTP conversation