The usually civilian or partly civilian executive control over the national military organization is exercised in democracies by an elected political leader as a member of government's Cabinet, usually known as a Minister of Defence.
(In presidential systems, such as the United States, the president is the commander-in-chief, and a the cabinet-level defense minister is second in command.) Subordinated to that position will be Secretaries for specific major operational divisions of the armed forces as a whole, such as those that provide general support services to the Armed Services, including their dependents. Third in the chain of command will be the heads of specific departmental agencies responsible for provision and management of specific skill and knowledge based service such as Strategy advice, Capability Development assessment, or Defence Science provision of research, design and development of technologies. Within each departmental agency will be found administrative branches responsible for further agency business specialization work. For example the Strategy agency may have one branch producing advice on operational concepts while another may provide advice on operational methods, and a third branch would provide the synthesis of these in the application of joint operations by Armed Services.
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Electronic mail, often abbreviated as e-mail, email, or eMail, is any method of creating, transmitting, or storing primarily text-based human communications with digital communications systems. Historically, a variety of electronic mail system designs evolved that were often incompatible or not interoperable. With the proliferation of the Internet since the early 1980s, however, the standardization efforts of Internet architects succeeded in promulgating a single standard based on the Simple Mail Transfer Protocol (SMTP), first published as Internet Standard 10 (RFC 821) in 1982.
Modern e-mail systems are based on a store-and-forward model in which e-mail computer server systems, accept, forward, or store messages on behalf of users, who only connect to the e-mail infrastructure with their personal computer or other network-enabled device for the duration of message transmission or retrieval to or from their designated server. Rarely is e-mail transmitted directly from one user's device to another's.
While, originally, e-mail consisted only of text messages composed in the ASCII character set, virtually any media format can be sent today, including attachments of audio and video clips.
Header fields
The message header usually includes at least the following fields:
From: The e-mail address, and optionally the name of the sender
To: The e-mail address[es], and optionally name[s] of the message's recipient[s]
Subject: A brief summary of the contents of the message
Date: The local time and date when the message was written
Note that the "To" field is not necessarily related to the addresses to which the message is delivered. The actual delivery list is supplied in the SMTP protocol, not extracted from the header content. The "To" field is similar to the greeting at the top of a conventional letter which is delivered according to the address on the outer envelope. Also note that the "From" field does not have to be the real sender of the e-mail message. It is very easy to fake the "From" field and let a message seem to be from any mail address. It is possible to digitally sign e-mail, which is much harder to fake. Some Internet service providers do not relay e-mail claiming to come from a domain not hosted by them, but very few (if any) check to make sure that the person or even e-mail address named in the "From" field is the one associated with the connection. Some Internet service providers apply e-mail authentication systems to e-mail being sent through their MTA to allow other MTAs to detect forged spam that might appear to come from them.
Other common header fields include (see RFC 4021 or RFC 2076 for more):
Bcc: Blind Carbon Copy
Cc: Carbon copy
Content-Type: Information about how the message has to be displayed, usually a MIME type
In-Reply-To: Message-ID of the message that this is a reply to.
Received: Tracking information generated by mail servers that have previously handled a message
References: Message-ID of the message that this is a reply to, and the message-id of this message, etc.
Reply-To: Address that should be used to reply to the sender.
X-Face: Small icon.
Many e-mail clients present "Bcc" (Blind carbon copy, recipients not visible in the "To" field) as a header field. Different protocols are used to deal with the "Bcc" field; at times the entire field is removed, whereas other times the field remains but the addresses therein are removed. Addresses added as "Bcc" are only added to the SMTP delivery list, and do not get included in the message data.
IANA maintains a list of standard header field
