E-Mail, in a format used by some traditional Internet standards, is one method commonly used for communication over computer networks. (For some other ways of delivering electronic messages, see online chat programs. The E-Mail systems more commonly used with terminal connections section may also have some information.

Internet E-Mail

This software is meant to be able to handle E-Mail on the Internet, generally meaning that it supports standard communications protocols.

Client software
Thunderbird
Mozilla's Thunderbird may be the most well known stand-alone E-Mail client (that isn't released by a creator/vendor of an operating system).
Suite software based on Netscape/Mozilla code
IceApe and SeaMonkey would logically be considered the modern free successors to the Mozilla Application Suite and the Netscape 4 Communicator software, as the development of these projects have used computer programming code that was updated from those older projects.
Other E-Mail software provided by the Operating System's creator/vendor
Options provided by Microsoft
Some options include: Windows Mail, Outlook Express, Office Outlook (and Outlook Web Access)
Mail server software
Sendmail

Sendmail is for operating systems that are designed to be similar to Unix.

qmail

D.J. Bernstein's qmail works for several platforms that are designed to be similar to Unix.

Postfix

From the main page: this is “an alternative to the widely-used Sendmail program.” “Postfix attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different.”

Apparently this does not have a Windows port: a guide at http://andrewbevitt.com/2007/06/28/postfix-under-windows/ has information on using Postfix in Windows, but the method is to use a virtual machine.

OpenSMTPD

From the Calomel.org page about OpenSMTPD: “Its goal is to be a secure mailing daemon without the licensing restrictions of Postfix and without the added complexity of sendmail.”

hMailServer
The download page for hMailServer says “hMailServer source code, up until the latest 4.x-version, can be found on” the source code for hMailServer page. However, that does not seem to be true: The source code is available for version 4.4.2 builds 275, 279, and 283 from that page. They also offer a CD/DVD containing source code. The hMailServer download archive contains “4.4.2 - Build 283”, as well as versions 4.4.3, 4.4.4, 5.0, and newer versions, so there were newer versions and even more specifically newer 4.4.x versions where the source code page does not provide any downloads for.
Microsoft Exchange

This is software by Microsoft.

QWK

Before Internet usage became used or even understood by much of the general public, enthusiasts of computer usage would often use E-Mail provided by a “bulliten board system”. For further information about some software used to accomplish this, see the section on terminal-based software. Some BBS software supported a standard called QWK. The *.QWK files would contain new messages and the file could then be sent over the modem. The end user could then be offline (and not using up the phone lines for the BBS or the end user) while reading E-Mails and writing responses that could then be uploaded as a different *.QWK file.

QWK specification
QWK specification, another copy of QWK specification
SLMR and OLX (and SLMR)
“Silly Little Mail Reader” (“SLMR”)

SLMR was a popular piece of software for using interacting with mail offline with *.QWK files. Version 2.1a's file date was January 2, 1992, as seen on the Simtel archives (according to the copy of SLMR 2.1a shown on eunet.gb mirror). The documentation noted that on January 6, 1992 (which would be just a few days later), SLMR would become to properly to Mustang Software, Inc. SLMR 2.1a's licence says, “Do not send payments to us for use of SLMR. If you like the program you can either continue to use it in unregistered form or you may contact MSI for a Test-Drive copy of OLX, SLMR's new product derivative.” This would allow commercial usage, unlike OLX.

OLX
OLX for DOS

This was a popular piece of software for interacting with mail offline with *.QWK files. Here was its license: “Off-Line Xpress Test-Drive is licensed for individual personal use and evaluation for an unlimited time. Use and evaluation by businesses, corporations or individuals in a commercial venture is limited to 60 days, after which time the REGISTERED version of Off-Line Xpress must be purchased or the use of Off-Line Xpress Test-Drive must be discontinued.”

OLX has been sold by Mustang Software to Santronics, along with other Mustang Software products such as the “WildCat!” BBS software (which has since been renamed to “WINServer” and to “Wildcat! Interactive Net Server”).

Therefore, the following hyperlink points to a file that is deemed to be authentic: OLX-TD (OFF-LINE XPRESS v2.1 TEST-DRIVE) (obtained from Santronics).

http://processlist.com/info/olx-td.html suggests the file size is normally a larger 228,665 byte file. Such a file has been located and seems to be created using an older version of Yoshi's LHA program. ftp://ftp.simtel.net/pub/simtelnet/msdos/mailnews/olx21-td.zip ftp://ftp.mpoli.fi/pub/software/DOS/OFFLINE/OLX-TD21.ZIP SOTL.org had: File listing of area 33 which required a free registration which includes OLX-TD.EXE. This hasn't yet been verified as authentic.

OLX for Windows

Off-Line Xpress (OLX) is currently available as version 3.x for Windows. Older releases include OLX-TD 2.1, and the product's original name of Silly Little Mail Reader (which was meant to be abbreviated as SLMR and pronounced as Slimer).

Here are some potential downloads that may work well: ftp://ftp.simtel.net/pub/simtelnet/msdos/mailnews/slmr21a.zip