|How Email Works||Back to Home Page|
HOW DOES MY COMPUTER SEND AND RECEIVE MESSAGES?
The process is actually very similar to the regular Post Office system, except that it happens much faster. The two basic components of Email transport are Simple Mail Transport Protocol (SMTP) and Post Office Protocol (POP3).
The process of sending and receiving Email is illustrated by this diagram:
The simplest mail to send and deliver is local mail. When you drop a letter in the mail box, the local Post Office (synonymous with mail.yellowhead.com) collects the mail and places it in the recipient's mail box where it stays until his/her Email program picks it up. When you check your mail, your Email program uses what is called POP version 3 (POP3) to pick up your mail and move it to your PC. For local mail delivery, there are no restrictions as to who can converse with the mail server. Email programs usually have an added function that allows you to essentially read your mail and put it back in the mail box (leave mail on server).
Local delivery (i.e. to email@example.com) occurs immediately. Good email programs will tell you if local delivery cannot be made without resorting to sending you a message later.
When somebody sends you mail, they actually send the mail to their own Post Office and ask it to forward that mail to the recipient's Post Office. The Post Office or Mail server on the other end uses the SMTP protocol to deliver the mail to your mail server (mail.yellowhead.com). Your Mail server puts mail in your "mail drop", where it stays until your Email program picks it up. When you check your mail, your Email program uses POP3 to pick up your mail and move it to your PC.
The SMTP and POP3 protocols are very mature. In the early days, there were virtually no restrictions on their use. Now however, unscrupulous individuals have taken to abusing the system to spread Spam or junk mail, and mail servers have had to impose limitations to curb the abuse. For example, our mail server will not receive mail that has a return address from a non-existent domain (e.g. firstname.lastname@example.org). Nor will it forward mail to another server that does not come from an IP address in our own domain.
When you send an Email message to someone, your Email program (e.g. Eudora) uses SMTP to send the mail to the local Post Office (e.g. mail.yellowhead.com). The server queues the outgoing messages on its disk, and when it has the time it will forward the mail to your addressee's Post Office, usually by means of the SMTP protocol. It will not accept messages that come from an IP address outside of the yellowhead.com domain. For example, you cannot log onto the Internet using a Telus account and use the yellowhead.com mail server to send mail. You must use the Sendmail or SMTP server that belongs to the network you are logged into.
What happens if the mail server cannot find the mail box of the person to whom you addressed the mail? As long as you have the correct return address on your mail, you will be informed of any problems. If the return address is incorrect, the same thing happens as in a regular Post Office; it goes to the dead letter box. Dead letters get sent to email@example.com where an attempt is made to find the sender. Mail servers do fail from time to time, and as long as the letter is addressed to a legitimate domain, our mail server will try for 2 hours to send the mail and then send you a message about the difficulties that it is having. It will continue to try and deliver the message for 3 days before it gives up and sends you another message to that effect.
Most mail programs allow the user to send moderate sized attachments along with the mail text. How that is accomplished varies from program to program, but the important part is to send it as a separate file, and not to include it in the body of the message. Some mail programs (in particular Microsoft programs), allow different objects such as attachments to be included in the body of the text. This is a non-standard approach and is not supported by most email programs. Most mail programs use the MIME standard rather than the BinHex standard.
The yellowhead.com mail server imposes a maximum message size of 3 MB. Allowing 1/4 MB for the message itself, that leaves the maximum attachment size at 2.75 MB. Some types of files such as documents (.doc) and spreadsheets (.xls) have very inefficient file formats and compress to as little as 20% of their original size. You will save yourself and your recipient some time if you compress these files using a program such as WINZIP or PKZIP before sending them as an attachment. Other files such as .jpeg or .gif files are already highly compressed, and it would be a waste of time to compress them further.| Technical Explanation For Sending Mail
| Technical Explanation For Receiving Mail
Excerpts taken from Eudora Pro v3.0 User Manual © 1996 Qualcomm, Inc.
| Home Page