Rebex Secure Mail
SMTP, IMAP, EWS, POP3, S/MIME .NET library
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Standards and platform support
On this page:
- 100% managed .NET code with no external dependencies
- .NET 5.0
- .NET Framework on Windows
- .NET Core
- .NET Standard
- Mono on Linux
- Xamarin.iOS
- Xamarin.Android
- Xamarin.Mac
- Universal Windows Platform
- ASP.NET
- Legacy platform support
- .NET Compact Framework
- Internet RFC specifications
- TLS/SSL versions
- MIME, S/MIME, Outlook MSG and other email formats
- FIPS 140-2 mode
- Proxy servers
- Authentication methods
- Exchange Web Services (EWS)
- SMTP, IMAP, POP3 Extensions
- Cryptographic algorithms
- Scripting platforms
- Other technologies
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.
.NET 5.0

Rebex Secure Mail supports .NET 5.0 on all supported operating systems:
- 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)
.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 2019
- Microsoft Visual Studio 2017
- Microsoft Visual Studio 2015
- Microsoft Visual Studio 2013
- Microsoft Visual Studio 2012
Supported Windows versions:
- Windows 10
- Windows Server 2019
- Windows Server 2016
- Windows 8.1
- Windows Server 2012 R2
- Windows Server 2012
- Windows Server 2008 R2 (end-of-life platform)
- Windows Server 2008 (end-of-life platform)
- Windows 7 (end-of-life platform)
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 frameworks:
- .NET Core 3.1
- .NET Core 3.0 (end-of-life platform)
- .NET Core 2.2 (end-of-life platform)
- .NET Core 2.1
- .NET Core 2.0 (end-of-life platform)
Supported operating systems:
- Windows 10
- Windows 10 IoT (.NET Core 3.1 and 3.0)
- Windows 8.1
- Windows 7 SP1 (end-of-life platform)
- Windows Server 2008 R2 SP1 (end-of-life platform)
- Linux
- macOS
.NET Standard
Rebex Secure Mail supports .NET Standard 2.1 on .NET Core 3.1 (or higher). It also supports .NET Standard 2.0 on .NET Core 2.0 (or higher), Mono 5.14 (or higher), Xamarin.Android 9.1 (or higher) and Xamarin.iOS 12.3 (or higher). Support for .NET Standard 2.0 on Universal Windows Platform (build 16299 or higher) and Xamarin.Mac (version 5.0 or higher) is experimental.
Xamarin.iOS

- Visual Studio with Xamarin.iOS 12.3 or later
- iOS 10.2 or later
To target Xamarin.iOS platform, reference .NET Standard 2.0 binaries.
Xamarin.Android

- Visual Studio with Xamarin.Android 9.1 or later
To target Xamarin.Android platform, reference .NET Standard 2.0 binaries.
Xamarin.Mac

Rebex Secure Mail features experimental support for Xamarin.Mac 5.0 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 via .NET Standard 2.0.
Supported platforms:
- 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
Supported operating systems:
- Windows Embedded Compact 2013
- Windows Embedded Compact 7
Known to work on the following end-of-life platforms:
- 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.
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:
- RFC 1847
- RFC 2045
- RFC 2046
- RFC 2047
- RFC 2048
- RFC 2049
- RFC 2822
- RFC 3217
- RFC 3370
- RFC 3565
- RFC 3850
- RFC 3851
- RFC 3852
- RFC 8017 (3447)
TLS RFCs:
- RFC 2246 (TLS 1.0)
- RFC 4346 (TLS 1.1)
- RFC 5246 (TLS 1.2)
- RFC 8446 (TLS 1.3)
- RFC 5746 (TLS Renegotiation Indication Extension)
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/li>
- 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, setRebex.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
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
SMTP, IMAP, POP3 Extensions
SMTP extensions
Pipelining
- Sending SMTP commands in groupsChunking
- Enhanced method of sending message dataBinary MIME
8bit MIME
ETRN
- Enhanced turn command supportEnhanced status codes
DSN
- Delivery status notificationsMessage size declaration
STARTTLS
- Explicit TLS/SSL support
IMAP extensions
UIDPLUS
- StoreMessage method will return unique ID of the message at the serverLITERAL+
- The LITERAL+ extension permits IMAP clients to send commands containing 8-bit or unicode arguments without asking the server's permission to send each argumentIDLE/DONE
command supportCOMPRESS=DEFLATE
- Deflate compression extension supportXLIST
- Extended list - created by Google for GMail IMAP - used to retrieve folder purpose infoSTARTTLS
- Explicit TLS/SSL supportPREAUTH
- Password-less authentication
POP3 extensions
APOP
- an enhanced authorization methodSTLS
- Explicit TLS/SSL support
Cryptographic algorithms
- Diffie-Hellman key exchange
- RSA
- DSA (with support of non-standard key sizes of up to 4096 bits)
- DES (legacy)
- Tripple DES
- ArcTwo (compatible with RC2, legacy)
- ArcFour (compatible with RC4)
- AES
- MD5 (legacy)
- SHA-1
- SHA-2 (SHA-256, SHA-384, SHA-512)
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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