Electronic Mail

Forwrding messages : forward the received message to third party with or without extra information

Handling mailboxes : user agent create and maintain two mailboxes inbox and outbox.

User agents are of 2 typesCommand based : mail, pine and elmGUI : netscape, outlook, eudora etc.

The main limitation of Email is that it can send messages only in 7 bit ASCII format. Therefore it can not be used for languages that are not supported by 7 bit ASCII characters ( French, german, Russian, Chinese etc.)

The another limitation is that it can not be used to send binary files or video or audio data.

MIME ( Multipurpose Internet Mail Extensions ) is a supplementary protocol that allows non ASCII data to be sent through email. The MIME transform non ASCII data at the sender side to ASCII data and at the receiving side it is transformed back to the original data.

MIME Header

Data Types in MIME

Content-Transfer-Encoding

Mail Transfer Agent (MTA): SMTP

• The actual mail transfer is done through MTA

Commands Commands are sent from the client to the server. The fonnat of a command is shown in Figure 26.18. It consists of a keyword followed by zero or more arguments.SMTP defines 14 commands. The first five are mandatory; every implementation must support these five commands. The next three are often used and highly recommended. The last six are seldom used.

CommandsHELO Sender's host nameMAIL FROM Sender of the messageRCPTTO Intended recipient of the messageDATA Body of the mailQUITRSETVRFY Name of recipient to be verifiedNOOPTURNEXPN Mailing list to be expandedHELP Command nameSEND FROM Intended recipient of the messageSMOLFROM Intended recipient of the messageSMALFROM Intended recipient of the message

Responses Responses are sent from the server to the client. A response is a threedigit code that may be followed by additional textual information.

As the table shows, responses are divided into four categories. The leftmost digit of the code (2, 3, 4, and 5) defines the category.

Code DescriptionPositive Completion Reply211 System status or help reply214 Help message220 Service ready221 Service closing transmission channel250 Request command completed251 User not local; the message will be forwardedPositive Intermediate Reply354 Start mail inputTransient Negative Completion Reply421 Service not available450 Mailbox not available451 Command aborted: local error452 Command aborted: insufficient storagePermanent Negative Completion Reply500 Syntax error; unrecognized command501 Syntax error in parameters or arguments502 Command not implemented503 Bad sequence of commands504 Command temporarily not implemented550 Command is not executed; mailbox unavailable551 User not local552 Requested action aborted; exceeded storage location553 Requested action not taken; mailbox name not allowed554 Transaction failed

POP3 is deficient in several ways. It does not allow the user to organize her mail on the server; the user cannot have different folders on the server. (Of course, the user can create folders on her own computer.) In addition, POP3 does not allow the user to partiall check the contents of the mail before downloading.

Internet Mail Access Protocol, version 4 (IMAP4)

IMAP4 is similar to POP3, but has more features with extra functions

•A user can check the email header prior to downloading•A user can search the contents of the email for a specific string of characters prior to downloading•A user can create, delete, or rename mailboxes on the mail server•A user can create a hierarchy of mailboxes in a folder for email storage

POP3 and IMAP4

Electronic Mail (1,2)

• When the sender and the receiver of an e-mail are on the same system, we need only two user agents.

• When the sender and the receiver of an e-mail are on different systems, we need two UAs and a pair of MTAs (client and server).

Electronic Mail (3)

• When the sender is connected to the mail server via a LAN or a WAN, we need two UAs and two pairs of MTAs (client and server).

Electronic Mail (4)

• When both sender and receiver are connected to the mail server via a LAN or a WAN, we need two UAs, two pairs of MTAs and a pair of MAAs. This is the most common situation today.