Email forwarding - AbsoluteAstronomy.com

Email forwarding generically refers to the operation of re-sending an email message

E-mail

Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...

delivered to one email address on to a possibly different email address. The term forwarding has no specific technical meaning.
Users and administrators of email systems use the same term when speaking of both server-based and client-based forwarding.

Email forwarding can also redirect mail going to one address and send it to one or several other addresses. Vice versa, email items going to several different addresses can converge via forwarding to end up in a single address in-box.

Server-based forwarding

The domain name

Domain name

A domain name is an identification string that defines a realm of administrative autonomy, authority, or control in the Internet. Domain names are formed by the rules and procedures of the Domain Name System ....

(the part appearing to the right of @ in an email address) defines the target server(s)
for the corresponding class of addresses. A domain may also define backup servers; they have no mailboxes and forward messages without changing any part of their envelopes. By contrast, primary servers can deliver a message to a user's mailbox and/or forward it by changing some envelope addresses. ~/.forward files (see below) provide a typical example of server-based forwarding to different recipients.

Email administrators sometimes use the term redirection as a synonym for server-based email-forwarding to different recipients.

Because of spam, it is becoming increasingly difficult to reliably forward mail across different domains, and some recommend to avoid it if at all possible.

Uses of server-based forwarding to different recipients

Role-addresses : info, sales, postmaster, and similar names can appear to the left of @ in email addresses. An organization may forward messages intended for a given role to the personal address of the person(s) functioning in that role or office.
Vanity-addresses : Most domain name hosting facilities provide mail-facilities to forward mail to another email address such as the user's Internet Service Provider

Internet service provider

An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...

.
Dismissed addresses : When users change their email addresses, an administrator may set up forwarding from an old address to a current one in order to avoid losing messages.

Forwarding versus remailing

Plain message-forwarding changes the envelope recipient(s) and leaves the envelope sender field untouched. The "envelope sender" field does not equate to the From header which Email client software usually displays: it represents a field used in the early stages of the SMTP protocol, and subsequently saved as the Return-Path header. This field holds the address to which mail-systems must send bounce message

Bounce message

In the Internet's standard e-mail protocol SMTP, a bounce message, also called a Non-Delivery Report/Receipt , a Delivery Status Notification message, a Non-Delivery Notification or simply a bounce, is an automated electronic mail message from a mail system informing the sender of another...

s — reporting delivery-failure (or success) — if any.

By contrast, the terms remailing or redistribution can sometimes mean re-sending the message and also rewriting the "envelope sender" field. Electronic mailing list

Electronic mailing list

An electronic mailing list is a special usage of email that allows for widespread distribution of information to many Internet users. It is similar to a traditional mailing list — a list of names and addresses — as might be kept by an organization for sending publications to...

s furnish a typical example. Authors submit messages to a reflector that performs remailing to each list address. That way, bounce message

Bounce message

s (which report a failure delivering a message to any list- subscriber) will not reach the author of a message. However, annoying misconfigured vacation autoreplies do reach authors.

Typically, plain message-forwarding does alias-expansion, while proper message-forwarding, also named forwarding tout-court serves for mailing-lists. When additional modifications to the message are carried out, so as to rather resemble the action of a MUA submitting a new message, the term forwarding becomes deceptive and remailing seems more appropriate.

In the Sender Policy Framework

Sender Policy Framework

Sender Policy Framework is an email validation system designed to prevent email spam by detecting email spoofing, a common vulnerability, by verifying sender IP addresses. SPF allows administrators to specify which hosts are allowed to send mail from a given domain by creating a specific SPF...

(SPF), the domain-name in the envelope sender remains subject to policy restrictions. Therefore, SPF generally disallows plain message-forwarding. Intra domain redirection complies with SPF as long as the relevant servers share a consistent configuration. Mail servers that practise inter-domain message-forwarding may break SPF even if they don't implement SPF themselves, i.e. they neither apply SPF checks nor publish SPF records.
Sender Rewriting Scheme

Sender Rewriting Scheme

Sender Rewriting Scheme is a technique to re-mail an email message so that eventual Delivery Status Notifications can reach the original message sender...

provides for a generic forwarding mechanism compatible with SPF.

Automated client-based forwarding

Client forwarding can take place automatically using a non-interactive client such as a mail retrieval agent

Mail retrieval agent

A mail retrieval agent is a computer application that retrieves or fetches e-mail from a remote mail server and works with an mail delivery agent to deliver mail to a local or remote email mailbox. MRAs may be external applications by themselves or be built into a bigger application like an MUA...

. Although the retrieval agent uses a client protocol, this forwarding resembles server forwarding in that it keeps the same message-identity. Concerns about the envelope-sender apply.

Manual client-based forwarding

An end-user

End-user

Economics and commerce define an end user as the person who uses a product. The end user or consumer may differ from the person who purchases the product...

can manually forward a message using an email client. Forwarding inline quotes the message below the main text of the new message, and usually preserves original attachments as well as a choice of selected headers (e.g. the original From and Reply-To.) The recipient of a message forwarded this way may still be able to reply to the original message; the ability to do so depends on the presence of original headers and may imply manually copying and pasting the relevant destination addresses.

Forwarding as attachment prepares a MIME attachment (of type message/rfc822) that contains the full original message, including all headers and any attachment. Note that including all the headers discloses much information about the message, such as the servers that transmitted it and any client-tag added on the mailbox. The recipient of a message forwarded this way may be able to open the attached message and reply to it seamlessly.

This kind of forwarding actually constitutes a remailing from the points of view of the envelope-sender and of the recipient(s). The message identity also changes.

Historical development of email forwarding

RFC 821, Simple Mail Transfer Protocol, by Jonathan B. Postel in 1982, provided for a forward-path for each recipient, in the form of, for example, @USC-ISIE.ARPA, @USC-ISIF.ARPA: Q-Smith@ISI-VAXA.ARPA — an optional list of hosts and a required destination-mailbox. When the list of hosts existed, it served as a source-route, indicating that each host had to relay the mail to the next host on the list. Otherwise, in the case of insufficient destination-information but where the server knew the correct destination, it could take the responsibility to deliver the message by responding as follows:



      S: RCPT TO:

      R: 251 User not local; will forward to

The concept at that time envisaged the elements of the forward-path (source route) moving to the return-path (envelope sender) as a message got relayed from one SMTP server to another. Even if the system discouraged the use of source-routing,
dynamically building the return-path implied that the "envelope sender" information could not remain in its original form during forwarding. Thus RFC 821 did not originally allow plain message-forwarding.

The introduction of the MX record

MX record

A mail exchanger record is a type of resource record in the Domain Name System that specifies a mail server responsible for accepting email messages on behalf of a recipient's domain, and a preference value used to prioritize mail delivery if multiple mail servers are available...

made source-routing unnecessary. In 1989, RFC 1123 recommended accepting source-routing only for backward-compatibility. At that point, plain message forwarding became the recommended action for alias-expansion. In 2008, RFC 5321 still mentions that "systems may remove the return path and rebuild [it] as needed", taking into consideration that not doing so might inadvertently disclose sensitive information.
Actually, plain message-forwarding can be conveniently used for alias expansion within the same server or a set of coordinated servers.

`~/.forward` files

sendmail

Sendmail

Sendmail is a general purpose internetwork email routing facility that supports many kinds of mail-transfer and -delivery methods, including the Simple Mail Transfer Protocol used for email transport over the Internet....

, the reference SMTP implementation in the early 1980s, provided for ~/.forward files, which can store the target email-addresses for given users. One can configure some email-program filter

E-mail filtering

Email filtering is the processing of email to organize it according to specified criteria. Most often this refers to the automatic processing of incoming messages, but the term also applies to the intervention of human intelligence in addition to anti-spam techniques, and to outgoing emails as well...

s to automatically perform forwarding or replying actions immediately after receiving. Forward files can also contain shell script

Shell script

A shell script is a script written for the shell, or command line interpreter, of an operating system. It is often considered a simple domain-specific programming language...

s, which have become a source of many security problems. Formerly only trusted users could utilize the command-line switch for setting the envelope sender, -f arg; some systems disabled this feature for security reasons.

Email predates the formalization of client–server architectures in the 1990s.
Therefore, the distinction between client

Client (computing)

A client is an application or system that accesses a service made available by a server. The server is often on another computer system, in which case the client accesses the service by way of a network....

and server

Server (computing)

In the context of client-server architecture, a server is a computer program running to serve the requests of other programs, the "clients". Thus, the "server" performs some computational task on behalf of "clients"...

seems necessarily forced. The original distinction contrasted daemons

Daemon (computer software)

In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...

and user-controlled programs

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

which run on the same machine. The sendmail daemon used to run with root

Superuser

On many computer operating systems, the superuser is a special user account used for system administration. Depending on the operating system, the actual name of this account might be: root, administrator or supervisor....

privilege

Privilege (Computing)

In computing, privilege is defined as the delegation of authority over a computer system. A privilege is a permission to perform an action. Examples of various privileges include the ability to create a file in a directory, or to read or delete a file, access a device, or have read or write...

s so it could impersonate any user whose mail it had to manage. On the other hand, users can access their own individual mail-files and configuration files, including ~/.forward. Client programs may assist in editing the server configuration-files of a given user, thereby causing some confusion as to what role each program plays.

Virtual users

The term "virtual users" refers to email users who never log on a mail-server system and only access their mailboxes using remote clients. A mail-server program may work for both virtual and regular users, or it may require minor modifications to take advantage of the fact that virtual users frequently share the same system id. The latter circumstance allows the server program to implement some features more easily, as it doesn't have to obey system-access restrictions. The same principles of operations apply. However, virtual users have more difficulty in accessing their configuration files, for good or ill.

External links

sendmail and the .forward file

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