Rebex SMTP

SMTP, MIME, S/MIME libraries for .NET

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Authentication modes

Username and password

Password-based authentication is simple:

C#
VB.NET

// create client instance and connect
// ...

// log in
client.Login(username, password);

' create client instance and connect
' ...

' log in
client.Login(username, password)

Tip: To protect your credentials, use TLS/SSL.

CRAM-MD5 authentication

Some servers support CRAM-MD5 authentication mechanism. When communicating over an unencrypted channel, this is more secure than plain-text password authentication.

C#
VB.NET

// create client instance and connect
// ...

// log in using CRAM-MD5
client.Login(username, password, SmtpAuthentication.CramMD5);

' create SMTP client instance and connect
Dim client = New Rebex.Net.Smtp()
client.Connect(hostname, port)

' log in using CRAM-MD5
client.Login(username, password, SmtpAuthentication.CramMD5)

DIGEST-MD5 authentication

Some servers support DIGEST-MD5 authentication mechanism. When communicating over an unencrypted channel, this is more secure than plain-text password authentication.

C#
VB.NET

// create client instance and connect
// ...

// log in using DIGEST-MD5
client.Login(username, password, SmtpAuthentication.DigestMD5);

' create SMTP client instance and connect
Dim client = New Rebex.Net.Smtp()
client.Connect(hostname, port)

' log in using DIGEST-MD5
client.Login(username, password, SmtpAuthentication.DigestMD5)

OAuth 2.0 authentication

Most cloud-based SMTP servers (such as Gmail or Microsoft 365 / Outlook Online) support OAuth 2.0 authentication mechanism.

To log in using OAuth, an application (or its user) authenticates via the service provider's authentication server and receives an access token. Then it connects to the SMTP server and authenticates using the obtained token:

C#
VB.NET

// create client instance and connect
// ...

// authenticate using OAuth 2.0 access token
client.Login(userEmail, accessToken, SmtpAuthentication.OAuth20);

' connect to a server
Dim client = New Rebex.Net.Smtp()
client.Connect("smtp.gmail.com", SslMode.Implicit)

' authenticate using OAuth 2.0 access token
client.Login(userEmail, accessToken, SmtpAuthentication.OAuth20)

The process of obtaining the access token is slightly different for each provider and application type. For details, see the following articles:

Gmail: How to authenticate with Rebex Mail using OAuth 2.0
Office 365: How to register your app (for apps with signed-in users)
Office 365: How to use OAuth 2.0 with Rebex Mail (for apps with signed-in users)
Office 365 and IMAP/POP3 in unattended (app-only) mode (for services and daemons)

Client certificate authentication

Client certificates are an optional way to authenticate the client to the server. This is only possible when connecting/authenticating to a TLS/SSL-capable server. However, most servers still require authentication with a username and password even when client certificate authentication has taken place.

A certificate with an associated private key is needed for client authentication. Set Settings.SslClientCertificateRequestHandler property to an implementation of certificate request handler that is called when the server asks for client certificate.

a) Use the built-in StoreSearch handler, that searches the user's certificate store for a first suitable certificate:

C#
VB.NET

// create client instance
// ...

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.StoreSearch;

// connect to the server
client.Connect(hostname, SslMode.Explicit);

// authenticate (still needed in many cases)
if (!client.IsAuthenticated)
    client.Login(username, password);

' create client instance
' ...

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.StoreSearch

' connect to the server
client.Connect(hostname, SslMode.Explicit)

' authenticate (still needed in many cases)
If Not client.IsAuthenticated Then
    client.Login(username, password)
End If

b) Use the built-in PFX-based certificate request handler:

C#
VB.NET

// create client instance
// ...

// load a certificate chain from a .pfx/.p12 file
CertificateChain certificate = CertificateChain.LoadPfx(@"C:\MyData\MyCertificate.pfx", "password");

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.CreateRequestHandler(certificate);

// connect to the server
client.Connect(hostname, SslMode.Explicit);

// authenticate (still needed in many cases)
if (!client.IsAuthenticated)
    client.Login(username, password);

' create client instance
' ...

' load a certificate chain from a .pfx/.p12 file
Dim certificate As CertificateChain = CertificateChain.LoadPfx("C:\MyData\MyCertificate.pfx", "password")

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = CertificateRequestHandler.CreateRequestHandler(certificate)

' connect to the server
client.Connect(hostname, SslMode.Explicit)

' authenticate (still needed in many cases)
If Not client.IsAuthenticated Then
    client.Login(username, password)
End If

c) Write a custom handler, for example to load the certificate from a .pfx/.p12 file:

C#
VB.NET

private class MyCertRequestHandler : ICertificateRequestHandler
{
    // This method is called during TLS/SSL negotiation
    // when the server requests client certificate authentication
    public CertificateChain Request(TlsSocket socket, DistinguishedName[] issuers)
    {
        // provide a certificate loaded from a .pfx/.p12 file
        return CertificateChain.LoadPfx(clientCertPath, clientCertPassword);
    }
}

Private Class MyCertRequestHandler
    Implements ICertificateRequestHandler
    ' This method is called during TLS/SSL negotiation
    ' when the server requests client certificate authentication
    Public Function Request(socket As TlsSocket, issuers As DistinguishedName()) As CertificateChain Implements ICertificateRequestHandler.Request
        ' provide a certificate loaded from a .pfx/.p12 file
        Return CertificateChain.LoadPfx(clientCertPath, clientCertPassword)
    End Function
End Class

Don't forget to register the handler:

C#
VB.NET

// create client instance
// ...

// set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = new MyCertRequestHandler();

// connect to the server
client.Connect(hostname, SslMode.Explicit);

// authenticate (still needed in many cases)
if (!client.IsAuthenticated)
    client.Login(username, password);

' create client instance
' ...

' set a certificate request handler
client.Settings.SslClientCertificateRequestHandler = New MyCertRequestHandler()

' connect to the server
client.Connect(hostname, SslMode.Explicit)

' authenticate (still needed in many cases)
If Not client.IsAuthenticated Then
    client.Login(username, password)
End If

GSSAPI

GSSAPI support in Rebex SMTP makes it possible to authenticate using Kerberos, NTLM or Negotiate authentication mechanisms, either in single sign-on mode or username/password(/domain)-based mode.

Note: GSSAPI is only supported on Windows platforms.

Single sign-on

With single sign-on, the current user can authenticate without having to enter his password. Single sign-on is only possible through NTLM or through GSSAPI with Kerberos, NTLM or Negotiate authentication mechanisms on servers that support them. Additionally, both the client and server machines might have to be part of the same domain (or a domain trust has to be established).

Note: Single sign-on is only supported on Windows platforms.

Kerberos authentication

In SMTP, Kerberos v5 is one of the supported GSSAPI authentication mechanisms.

// create client instance and connect
// ...

// initialize GSSAPI for Kerberos single sign-on
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Kerberos", null, null, null, null);

// log in using Kerberos single sign-on
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for Kerberos single sign-on
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Kerberos", Nothing, Nothing, Nothing, Nothing)

' log in using Kerberos single sign-on
client.Login(credentials)

GSSAPI/Kerberos with username/password/domain:

C#
VB.NET

// create client instance and connect
// ...

// initialize GSSAPI for Kerberos authentication
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Kerberos", null, username, password, domain);

// log in using Kerberos
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for Kerberos authentication
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Kerberos", Nothing, username, password, domain)

' log in using Kerberos
client.Login(credentials)

Note: Kerberos is only supported on Windows platforms. However, it's possible to authenticate Windows-based clients to Unix-based servers using Kerberos.

NTLM authentication

In SMTP, NTLM is one of the supported GSSAPI authentication mechanisms.

GSSAPI/NTLM in single sign-on mode:

C#
VB.NET

// create client instance and connect
// ...

// initialize GSSAPI for NTLM single sign-on
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Ntlm", null, null, null, null);

// log in using Kerberos single sign-on
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for NTLM single sign-on
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Ntlm", Nothing, Nothing, Nothing, Nothing)

' log in using Kerberos single sign-on
client.Login(credentials)

GSSAPI/NTLM with username/password/domain:

C#
VB.NET

// create client instance and connect
// ...

// initialize GSSAPI for NTLM authentication
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Ntlm", null, username, password, domain);

// log in using Kerberos
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for NTLM authentication
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Ntlm", Nothing, username, password, domain)

' log in using Kerberos
client.Login(credentials)

Alternatively, some servers support a stand-alone NTLM authentication.

(Standalone) NTLM with single sign-on:

C#
VB.NET

// create client instance and connect
// ...

// log in using NTLM single sign-on
client.Login(SmtpAuthentication.Ntlm);

' create SMTP client instance and connect
Dim client = New Rebex.Net.Smtp()
client.Connect(hostname, port)

' log in using NTLM single sign-on
client.Login(SmtpAuthentication.Ntlm)

(Standalone) NTLM with username/password:

C#
VB.NET

// create client instance and connect
// ...

// log in using NTLM
client.Login(username, password, SmtpAuthentication.Ntlm);

' create SMTP client instance and connect
Dim client = New Rebex.Net.Smtp()
client.Connect(hostname, port)

' log in using NTLM
client.Login(username, password, SmtpAuthentication.Ntlm)

Note: On non-Windows platforms (Linux, Android, macOS, iOS), NTLM is only available with NTLM plugin.

Negotiate authentication

In SMTP, Negotiate is one of the supported GSSAPI authentication mechanisms.

GSSAPI/Negotiate in single sign-on mode:

C#
VB.NET

// create client instance and connect
// ...

// initialize GSSAPI for Negotiate single sign-on
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Negotiate", null, null, null, null);

// log in using Kerberos single sign-on
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for Negotiate single sign-on
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Negotiate", Nothing, Nothing, Nothing, Nothing)

' log in using Kerberos single sign-on
client.Login(credentials)

GSSAPI/Negotiate with username/password/domain:

C#
VB.NET

// create client instance and connect
// ...

// initialize GSSAPI for Negotiate authentication
GssApiProvider credentials = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Negotiate", null, username, password, domain);

// log in using Kerberos
client.Login(credentials);

' create client instance and connect
' ...

' initialize GSSAPI for Negotiate authentication
Dim credentials As GssApiProvider = Rebex.Net.GssApiProvider.GetSspiProvider(
    "Negotiate", Nothing, username, password, domain)

' log in using Kerberos
client.Login(credentials)

Negotiate authentication mechanism automaticlly chooses whether to use Kerberos or NTLM.

Note: Negotiate is only supported on Windows platforms.