The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.
The X Window System is a windowing system for bitmap displays, common on Unix-like operating systems.
Pine is a freeware, text-based email client which was developed at the University of Washington. The first version was written in 1989, and announced to the public in March 1992. Source code was available for only the Unix version under a license written by the University of Washington. Pine is no longer under development, and has been replaced by the Alpine client, which is available under the Apache License.
The Maildir e-mail format is a common way of storing email messages in which each message is stored in a separate file with a unique name, and each mail folder is a file system directory. The local file system handles file locking as messages are added, moved and deleted. A major design goal of Maildir is to eliminate the need for program code to handle file locking and unlocking.
HCL Notes is a proprietary collaborative software platform for Unix, Windows, Linux and macOS, sold by HCLTech. The client application is called Notes while the server component is branded HCL Domino.
Webmail is an email service that can be accessed using a standard web browser. It contrasts with email service accessible through a specialised email client software. Additionally, many internet service providers (ISP) provide webmail as part of their internet service package. Similarly, some web hosting providers also provide webmail as a part of their hosting package.
Outlook Express, formerly known as Microsoft Internet Mail and News, is a discontinued email and news client included with Internet Explorer versions 3.0 through 6.0. As such, it was bundled with several versions of Microsoft Windows, from Windows 98 to Windows Server 2003, and was available for Windows 3.x, Windows NT 3.51, Windows 95, Mac System 7, Mac OS 8, and Mac OS 9. In Windows Vista, Outlook Express was superseded by Windows Mail.
Mail is an email client included by Apple Inc. with its operating systems macOS, iOS, iPadOS and watchOS. Mail grew out of NeXTMail, which was originally developed by NeXT as part of its NeXTSTEP operating system, after Apple's acquisition of NeXT in 1997.
Mutt is a text-based email client for Unix-like systems. It was originally written by Michael Elkins in 1995 and released under the GNU General Public License version 2 or any later version.
The Bat! is an email client for the Microsoft Windows operating system, developed by Moldovan software company Ritlabs. It is sold as shareware and offered in three editions: Home Edition, Professional Edition, and Voyager which is a portable version and is included with Professional Edition.
The MH Message Handling System is a free, open source e-mail client. It is different from almost all other mail reading systems in that, instead of a single program, it is made from several different programs which are designed to work from the command line provided by the shell on Unix-like operating systems. Another difference is that rather than storing multiple messages in a single file, messages each have their own separate file in a special directory. Taken together, these design choices mean that it is very easy and natural to script actions on mail messages using the normal shell scripting tools.
The following tables compare general and technical features of notable email client programs.
Hypermail is a free program for creating email archives, in the form of cross-referenced HTML documents. It takes a file in Unix mbox format and generates an HTML archive, complete with an index and various sorting options. It is commonly used for creating mailing list archives, but it can archive any collection of emails. Originally written in 1994 by Tom Gruber using Common Lisp, it was rewritten in C by Kevin Hughes for its initial public release in 1994.
CVSNT is a version control system compatible with and originally based on Concurrent Versions System (CVS), but whereas that was popular in the open-source world, CVSNT included features designed for developers working on commercial software including support for Windows, Active Directory authentication, reserved branches/locking, per-file access control lists and Unicode filenames. Also included in CVSNT were various RCS tools updated to work with more recent compilers and compatible with CVSNT.
mail is a command-line email client for Unix and Unix-like operating systems.
GNATS is the GNU project's issue-tracking software.
Alpine is a free software email client developed at the University of Washington.
Outlook.com, formerly Hotmail, is a free personal email service offered by Microsoft. This includes a webmail interface featuring mail, calendaring, contacts, and tasks services. Outlook can also be accessed via email clients using the IMAP or POP protocols.
Columbia MM is a computer program for reading email using a command-line interface. It was developed at Columbia University between 1984 and 1990, and is a Unix reimplementation of a 1978 TOPS-20 email program, also known as MM, which in turn was an update of an earlier program by Mike McMahon of SRI International. Columbia MM has also been built on other platforms, including DOS and VMS.
A text-based email client is an email client with its user interface being text-based, occupying a whole terminal screen. Other kind of email clients are GUI-based or Web-based, see Webmail.
