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75 lines
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==============
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Email Programs
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==============
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`Email <https://en.wikipedia.org/wiki/Email>`_ programs have become a communication standard for workers everywhere today. Email messages can include a wide range of information, from short chat-like interactions to lengthy and extensive text with complex document and image attachments. Whereas chat programs often assume that the sender and receiver are online at the same time, email programs use a `store and forward <https://en.wikipedia.org/wiki/Store_and_forward>`_ approach to ensure message delivery even when users are not connected simultaneously.
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Email operates on a client-server model. Users create or read their messages on some type of client program, although this software could be hosted on a network web server and accessed through a user's web browser rather than requiring a standalone email program to be installed on the client computer. Client programs typically access messages from the email server using either `Internet Message Access Protocol (IMAP) <https://en.wikipedia.org/wiki/Internet_Message_Access_Protocol>`_ or `Post Office Protocol (POP) <https://en.wikipedia.org/wiki/Post_Office_Protocol>`_. Client programs use `Simple Mail Transfer Protocol (SMTP) <https://en.wikipedia.org/wiki/Simple_Mail_Transfer_Protocol>`_ to send messages to email servers, while the servers themselves use SMTP for both sending and receiving.
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As with any client-server program, the email server must be reachable on a network segment with adequate bandwidth in order for the clients to exchange messages. If you have a choice, put your email server on one of your largest and most reliable network segments. Refer to this link for a comparison of email `Client Programs <https://en.wikipedia.org/wiki/Comparison_of_email_clients>`_, and visit this link for a comparison of email `Server Programs <https://en.wikipedia.org/wiki/Comparison_of_mail_servers>`_. The following list is not comprehensive or complete but represents a sample of the types of software that may be available for you to use as services on your mesh network. With one exception, only programs with open source licenses were included in this list, although proprietary email software can also be used.
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Citadel/UX
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----------
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Not only does Citadel provide email, but it is also a full-featured *groupware* suite with chat rooms, calendars and scheduling, contact address book, file sharing, forum posting, and many other features. It contains built-in implementations of the following server protocols: IMAP, POP3, SMTP, XMPP, and ManageSieve. Citadel also provides user self-registration, which minimizes the administrative overhead of managing email addresses on the server.
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Since a variety of features are bundled into a single application suite, Citadel is a less complicated and more integrated way to implement several network services at once by installing a single package capable of running on a lightweight `Raspberry Pi <https://en.wikipedia.org/wiki/Raspberry_Pi>`_ computer if necessary. Citadel's email services can be accessed using its browser-based webmail interface or from a separate email client program on a remote computer. For additional information about Citadel, visit this link: `Citadel <https://en.wikipedia.org/wiki/Citadel/UX>`_
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.. image:: _images/citadel.png
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:alt: Citadel Web Interface
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:align: center
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Open Source Email Server
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------------------------
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In order to implement an open source email server you will need to install several individual software packages, each of which will process one or more of the required email protocols. This is slightly more complicated than implementing a single groupware package such as the *Citadel* program described in the previous section. Protocols and example packages are described in the following lists.
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SMTP
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In order to implement an email server you will need to select a software package to handle the Simple Mail Transfer Protocol. You can select one of the example open source packages from the list below, or you can implement another SMTP agent of your choice.
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* `Sendmail <https://en.wikipedia.org/wiki/Sendmail>`_ is the original legacy SMTP server that is still used today, although one of the newer programs below is often chosen for its ease of configuration and added security features.
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* `Exim <https://en.wikipedia.org/wiki/Exim>`_ is the default SMTP server in Debian Linux, is well-documented, having many configurable features, and it runs from a single executable program.
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* `Postfix <https://en.wikipedia.org/wiki/Postfix_(software)>`_ is the default SMTP server in Ubuntu Linux and MacOS, with many integration and security features, and it runs a series of parallelized programs for improved performance.
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IMAP and POP3
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In order for email clients to retrieve their messages you will need to select a software package to handle IMAP and POP3 communication. You can select the example open source package below or you can implement another IMAP/POP3 package of your choice.
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* `Dovecot <https://en.wikipedia.org/wiki/Dovecot_(software)>`_ is one of the most popular IMAP and POP3 servers for open source email systems, being found on more than 2/3 of the email servers across the Internet.
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You will need to have detailed knowledge and skills when building your own open source email server, with the advantage of having complete control over everything on the system. There is some administrative overhead for creating and maintaining all user email accounts as well as handling other management tasks on your system. Using these open source software packages, it is possible to build a very robust email server that is capable of running on a small portable computer like a `Raspberry Pi <https://en.wikipedia.org/wiki/Raspberry_Pi>`_.
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.. image:: _images/thunderbird.png
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:alt: Thunderbird IMAP Client
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:align: center
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Using WinLink to Send Email
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---------------------------
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Although it is not typically used as a TCP/IP network application, many operators are already familiar with `WinLink 2000 <https://en.wikipedia.org/wiki/Winlink>`_ for sending message traffic between WinLink computers across amateur radio frequencies. It is possible to configure *Winlink Express* and Telnet Post Office or Telnet P2P for sending email with attachments across a mesh network.
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You will need a stable Microsoft Windows computer with plenty of memory to run this system (8GB recommended). The maximum attachment size is currently 5MB per message as compared to the 100KB limitation on HF and Packet RMS stations. Refer to the information below for details about specific network settings and procedures for configuring Winlink over AREDN |trade|. Additional information compiled by Orv Beach W6BI can be found in the `document linked here <https://www.arednmesh.org/sites/default/files/Configuring%20Winlink%20Express.pdf>`_.
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.. image:: _images/winlink.png
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:alt: Winlink Interface
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:align: center
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Example Email Service Comparison
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Platform abbreviations:
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win=MS Windows, mac=Apple, lin=Linux, rpi=Raspberry Pi
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========== ================== ============ ================= ======
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Program Features Network Load Platform Effort
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========== ================== ============ ================= ======
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Citadel groupware, webmail small lin/mac/rpi easy
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Open Email client-server small lin/mac/rpi expert
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WinLink email, attachments small win (proprietary) medium
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========== ================== ============ ================= ======
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