Rebex Secure Mail

SMTP, IMAP, EWS, POP3, S/MIME .NET library

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Standards and platform support

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100% managed .NET code with no external dependencies 

  • Rebex Secure Mail 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 6.0/5.0 

Rebex Secure Mail supports .NET 6.0 and .NET 5.0 on all supported operating systems:

  • Windows 11 (x64, x86, ARM64)
  • Windows 10 (x64, x86, ARM64)
  • Windows Server 2019 (x64, x86)
  • Windows Server 2016 (x64, x86)
  • Windows Server 2012 R2 (x64, x86)
  • Windows 8.1 (x64, x86)
  • Windows 7 SP1 (x64, x86) (end-of-life platform)
  • Linux (x64, ARM64, ARM32)
  • macOS (x64, ARM64)

Please note that support for the following .NET 6.0 platforms is currently in preview mode:

  • Android (x64, ARM64, ARM32)
  • iOS/tvOS (x64, ARM64, ARM32)

We will fully support Android and iOS/tvOS platforms as soon as the corresponding .NET 6.0 update is published.

.NET Framework on Windows 

Rebex Secure Mail supports the following .NET Framework platforms:

  • .NET Framework 4.8
  • .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 Secure Mail is also known to work on Windows 8, Windows Vista, 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 Secure Mail on these operating systems.

.NET Core 

Rebex Secure Mail supports .NET Core platform.

Supported versions:

Supported operating systems:

.NET Standard 

Rebex Secure Mail supports .NET Standard 2.1 on .NET 6.0, .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. Support for .NET Standard 2.0 on Universal Windows Platform (build 16299 or higher) and Xamarin.Mac (version 6.0 or higher) is experimental.

Mono on Linux 

Rebex Secure Mail supports Mono 5.x/6.x 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.

Xamarin.Mac 

Rebex Secure Mail features experimental support for Xamarin.Mac 6.0 or later via .NET Standard 2.0.

Universal Windows Platform 

Rebex Secure Mail features experimental support for Universal Windows Platform (UWP/UAP) applications on Windows 10 or Windows 11 via .NET Standard 2.0.

Supported platforms:

  • Windows 11
  • Windows 10 (version 1709 or higher)
  • Windows 10 IoT (version 1709 or higher)

In order to use Rebex Secure Mail on these platforms, make sure to specify at least Windows 10 Fall Creators Update (10.0; Build 16299) in your Universal Windows project.

ASP.NET 

Rebex Secure Mail 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 components in web environment can be found in How to use Rebex SFTP or FTP/SSL in ASP.NET knowledge base article. The article discusses SFTP and FTP/SSL, but implications for Rebex Secure Mail components are the same.

Legacy platform support 

Legacy edition of Rebex Secure Mail 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 Secure Mail 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 platforms:

  • 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 Secure Mail 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 Secure Mail aims to implement the following Internet standards:

SMTP RFCs:

POP3 RFCs:

IMAP 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 Secure Mail 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 Secure Mail 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 Secure Mail.

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.

POP3 authentication methods

  • Auto - automatically choose the best method available.
  • Plain - RFC 2595 plaintext authentication.
  • NTLM - Integrated Microsoft Windows Authentication.
  • GSSAPI - GSSAPI authentication (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.
  • ClearText - Legacy plaintext USER/PASS login.
  • APOP - POP3-specific challenge-response authentication.

IMAP 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.
  • ClearText - Legacy plaintext LOGIN command.
  • External - RFC 4422 - if the client already established its credentials via external means
See Authentication section to see how to select a specific authentication method.
Note: GSSAPI and NTLM authentication methods are only supported on Windows.

Exchange Web Services (EWS) 

The following servers currently support Exchange Web Services (EWS):

  • Microsoft Exchange 2007 and all service packs
  • Microsoft Exchange 2010 and all service packs
  • Microsoft Exchange 2013 and all service packs
  • Microsoft Exchange 2016 and all service packs
  • Microsoft Office 365 online servers
Note: Hotmail, Live.com and Outlook.com servers do not support EWS.

SMTP, IMAP, POP3 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

IMAP extensions

  • UIDPLUS - StoreMessage method will return unique ID of the message at the server
  • LITERAL+ - The LITERAL+ extension permits IMAP clients to send commands containing 8-bit or unicode arguments without asking the server's permission to send each argument
  • IDLE/DONE command support
  • COMPRESS=DEFLATE - Deflate compression extension support
  • XLIST - Extended list - created by Google for GMail IMAP - used to retrieve folder purpose info
  • STARTTLS - Explicit TLS/SSL support
  • PREAUTH - Password-less authentication

POP3 extensions

  • APOP - an enhanced authorization method
  • STLS - 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 Secure Mail 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 Secure Mail 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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