Rebex SMTP

SMTP, MIME, S/MIME libraries for .NET

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Standards and platform support

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100% managed .NET code with no external dependencies 

  • Rebex SMTP is written in 100% managed C# code.
  • No external dependencies. It only needs .NET.
  • No third-party or open source code used.
  • Fully CLS-compliant.
  • Supports any .NET language (C#, VB.NET, C++/CLI, etc.)
  • Platform-independent code. Same assemblies for both 32-bit and 64-bit applications and x86/x64, ARM32/ARM64 architectures.

.NET 9/8/7/6/5 

Rebex SMTP supports .NET 9, .NET 8 and .NET 7 on all supported operating systems:

  • Windows 11 (x64, ARM64)
  • Windows 10 (x64, x86, ARM64)
  • Windows Server 2022 (x64)
  • Windows Server 2019 (x64)
  • Windows Server 2016 (x64)
  • Windows Server 2012 (x64, x86)
  • Linux (x64, ARM64, ARM32)
  • macOS (x64, ARM64)
  • Android (x64, ARM64, ARM32)
  • iOS/tvOS (x64, ARM64, ARM32)

Rebex SMTP also supports .NET 6 and .NET 5 on all supported operating systems:

  • Windows 11 (x64, ARM64)
  • Windows 10 (x64, x86, ARM64)
  • Windows Server 2022 (x64)
  • Windows Server 2019 (x64)
  • Windows Server 2016 (x64)
  • Windows Server 2012 (x64, x86)
  • Windows 8.1 (x64, x86)
  • Windows 7 SP1 (x64, x86) (end-of-life platform)
  • Linux (x64, ARM64, ARM32)
  • macOS (x64, ARM64)
  • Android (x64, ARM64, ARM32)
  • iOS/tvOS (x64, ARM64, ARM32)

.NET Framework on Windows 

Rebex SMTP supports the following .NET Framework platforms:

  • .NET Framework 4.8.x
  • .NET Framework 4.7.x
  • .NET Framework 4.6.x
  • .NET Framework 4.5.x
  • .NET Framework 4.0
  • .NET Framework 3.5 SP1
  • Both 32-bit and 64-bit versions (same DLLs)

Legacy binaries for .NET Framework 2.0/3.0 are available as well.
For details and a trial version download link, see Support for Legacy Platforms.

Supported IDE versions:

  • Microsoft Visual Studio 2022
  • Microsoft Visual Studio 2019
  • Microsoft Visual Studio 2017
  • Microsoft Visual Studio 2015
  • Microsoft Visual Studio 2013
  • Microsoft Visual Studio 2012

Supported Windows versions:

Rebex SMTP is also known to work on Windows 8, Windows Vista SP1, Windows XP SP3, Windows Server 2003 R2, Windows Embedded Standard 2009 and Windows Embedded POSReady 2009. However, these platforms already reached end-of-life (most of them many years ago), which means we cannot guarantee support for Rebex SMTP on these operating systems.

.NET Core 

Rebex SMTP supports .NET Core platform.

Supported versions:

Supported operating systems:

.NET Standard 

Rebex SMTP supports .NET Standard 2.1 on .NET 9/8/7/6/5, .NET Core 3.1, Mono 6.10 (or higher), Xamarin.Android 10.1 (or higher) and Xamarin.iOS 13.10 (or higher). It also supports .NET Standard 2.0 on .NET Core 2.1.

Mono on Linux 

Rebex SMTP supports Mono 6.0 (or higher) on Linux and macOS.

Xamarin.iOS 

  • Visual Studio with Xamarin.iOS 13.10 or later
  • iOS 10.2 or later

To target Xamarin.iOS platform, reference .NET Standard 2.1 binaries.

Xamarin.Android 

  • Visual Studio with Xamarin.Android 10.1 or later

To target Xamarin.Android platform, reference .NET Standard 2.1 binaries.

ASP.NET 

Rebex SMTP can be used within an ASP.NET web page. However, you have to keep in mind specific features of the ASP.NET platform (it is stateless, multithreaded, it may be limited by MediumTrust security level...).

Further info on using Rebex libraries in web environment can be found in How to use Rebex SFTP or FTP in ASP.NET knowledge base article. The article discusses SFTP and FTP, but implications for Rebex SMTP libraries are the same.

Legacy platform support 

Legacy edition of Rebex SMTP for the following platforms is available as a separate product:

  • .NET Compact Framework 3.5/3.9
  • .NET Framework 2.0/3.0

Legacy platforms are platforms that are no longer covered by our Standard Support, but have not yet reached End of Life. For details, see the following KB articles:

.NET Compact Framework 

Rebex SMTP supports .NET Compact Framework on Microsoft's legacy embedded, handheld and mobile platforms based on Windows CE.

Supported frameworks:

  • .NET Compact Framework 3.9
  • .NET Compact Framework 3.5 (end-of-life)

Supported operating systems:

  • Windows Embedded Compact 2013

Known to work on the following end-of-life operating systems:

  • Windows Embedded Compact 7
  • Windows Embedded Handheld 6.5
  • Windows Embedded CE 6.0
  • Windows CE 5.0
  • Windows Mobile 5.0, 6.0, 6.1, 6.5

Please note .NET Compact Framework is a legacy platform, which means that Rebex SMTP for .NET CF is available as a separate product with different lifecycle and pricing.

Due to popular demand, we will continue to provide and maintain binaries for .NET Compact Framework 3.5 until 2023-10-10, as long as your legacy support contract remains active. However, support for these binaries is no longer included in the Rebex Support Contract.

Internet RFC specifications 

Rebex SMTP aims to implement the following Internet standards:

SMTP RFCs:

MIME, S/MIME and CMS RFCs:

TLS RFCs:

Other RFCs:

While we tried hard to be compliant with relevant RFCs, current practice in Internet mail often deviates from these standards. In fact, sometimes being compliant can severely hinder interoperability with existing software. Interoperability was therefore the highest priority.

For example, being compliant with RFC 2231 would lead to incompatibility with Microsoft Outlook, Outlook Express, and possibly a number of other mail agents as well. So even though Rebex SMTP is able to parse RFC2231-compliant messages, it does not produce them.)

TLS/SSL versions 

Supported TLS/SSL protocol versions:

  • TLS 1.3 (not available on .NET Compact Framework)
  • TLS 1.2
  • TLS 1.1
  • TLS 1.0
  • SSL 3.0

MIME, S/MIME, Outlook MSG and other email formats 

Supported e-mail message formats:

  • RFC 822/2822/5322 - Internet Message Format
  • MIME - Multipurpose Internet Mail Extensions
  • Outlook MSG - native format used by Microsoft Outlook
  • MS-TNEF/winmail.dat - Microsoft's proprietary email extension format

Supported attachment subformats:

  • UUEncode
  • Embedded OLE objects within TNEF/winmail.dat e-mail messages
  • Application-specific OLE attachments in MSG e-mail messages
  • Base64, Quoted-Printable, 7bit, 8bit and binary entities in MIME e-mail messages

FIPS 140-2 mode 

Rebex SMTP can operate in a FIPS 140-2 compliant mode. When enabled, only FIPS-approved cryptographic modules from MS CryptoAPI are used. FIPS mode is activated automatically when Windows is running in FIPS mode. To switch on the FIPS mode manually, set
Rebex.Security.Cryptography.CryptoHelper.UseFipsAlgorithmsOnly to true.

Proxy servers 

  • SOCKS4
  • SOCKS4a (Socks4 with domain name resolving)
  • SOCKS5
  • HTTP proxies with CONNECT method support

Learn how to use proxy servers in Rebex SMTP.

Authentication methods 

SMTP authentication methods

  • Auto - automatically choose the best method available.
  • Plain - RFC 2595 plaintext authentication.
  • NTLM - Integrated Microsoft Windows Authentication.
  • GSSAPI - currently supported mechanisms are Kerberos, NTLM and SPNEGO/Negotiate.
  • Digest-MD5 - RFC 2831 DIGEST-MD5 authentication.
  • CRAM-MD5 - RFC 2195 CRAM-MD5 authentication.
  • Login - 'LOGIN' plaintext authentication.
See Authentication section to see how to select a specific authentication method.
Note: GSSAPI and NTLM authentication methods are only supported on Windows.

SMTP Extensions 

SMTP extensions

  • Pipelining - Sending SMTP commands in groups
  • Chunking - Enhanced method of sending message data
  • Binary MIME
  • 8bit MIME
  • ETRN - Enhanced turn command support
  • Enhanced status codes
  • DSN - Delivery status notifications
  • Message size declaration
  • STARTTLS - Explicit TLS/SSL support

Cryptographic algorithms 

  • RSA
  • Ed25519
  • ECDSA
  • DSA (legacy)
  • Diffie-Hellman key exchange
  • Elliptic curve Diffie-Hellman key exchange (ECDH)
  • AES
  • AES/GCM
  • ChaCha20/Poly1305
  • Tripple DES
  • Twofish
  • DES (legacy)
  • Blowfish (legacy)
  • ArcFour (compatible with RC4, legacy)
  • ArcTwo (compatible with RC2, legacy)
  • SHA-2 (SHA-256, SHA-384, SHA-512)
  • SHA-1
  • MD5 (legacy)
  • MD4 (legacy)

Learn more about TLS/SSL security standards details.
Learn more about S/MIME security standards details.

Scripting platforms 

Rebex SMTP is designed primary for use in .NET framework applications. However, it can be used in several other scripting and hosting environments:

  • Microsoft SQL Server SSIS packages
  • Microsoft SQL Server SQL CLR stored procedures
  • Microsoft PowerShell scripts

See how to use Rebex SMTP in SSIS, SQL CLR and PowerShell environments.

Other technologies 

  • IPv4 and IPv6 support
  • Kerberos v5, NTLM and Negotiate (see more about GSSAPI authentication) on Windows

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