MTA, MDA what r they actually?

pippet · 07-21-2004, 11:32 PM

Howto says that

" MTA - Mail Transfer Agent. Accepts mail from other MTAs and mail users (you and I).

MDA - Mail Delivery Agent. Accepts inbound mail from an MTA and delivers it to the appropriate user on the local machine. "
I am confused of the role of MDA. Can u give me an example?

fr0zen · 07-22-2004, 12:15 AM

Okay, there are actually 3 pieces to the puzzle: MTA's, MDA's and MUA's.

MTA - Mail Transfer Agent
MDA - Mail Delivery Agent
MUA - Mail User Agent

The MUA starts the process. This is the program that sends the message. Thunderbird, Ximian are good examples of MUA's. These programs can both send a message, and receive inbound messages for local store.

When a MUA sends a message, it goes to the MTA. The MTA has a choice, it can either deliver the message, if the recipient is "local", or, it can forward the message to the next MTA, which then has to "ask" the same question. This goes on "forever" (servers do check for looping, though), until the final destination is reached. (Actually, modern servers are supposed to send directly to the destination server now, so usually there's only 1 or 2 transfers)

So now, lets assume that the final MTA has been contacted and is now receiving the message. Through a process dependent to the server itself, it will somehow store the message. This can be as simple as saving the message into a directory (eg: /var/spool/user). At this point, the process of saving to that spool directory, the server becomes the MDA, that is, it's deliverying the message to the user's spool.

Of course, this is only partially true for this type of mail system. What about POP3 users? Even for pop3, the message needs to be stored into a spool on the server. This is done by the MDA. Then, POP3 becomes the remote MDA to send the message to the MUA, which then, in turn is a local MDA, for local storage.

I'll try and diagram this out:

Code:

MUA -----> MTA 1 
                ----->  MTA 2  
                        (change role to MDA?)
                            + Send to some other program, like procmail
                            + Or (like sendmail) save it directly to a spool (/var/spool/user)

A good way to understand this is to think of the Postal system. Postal carriers (people) pick up mail and take it to the Post office, which then transports it to another post office, for local delivery. Then, a postal carrier becomes to the "delivery agent", by bringing the mail right to your house. Your mailbox is your MUA.

pippet · 07-22-2004, 02:26 AM

ok, i got it.
can u tell me what is imap?
how does it differ from pop3?

BIACS · 07-22-2004, 07:12 AM

Basically, imap will give you the ability to keep a copy of the mail on the mail server so you can still download it from another location. The best example I can give you is if you have the ability to check your mail on your ISP's web site (Comcast for example). Any messages that you don't delete while using their web mail interface will still be available when you get home. Same if you are using a client while at work to check your mail, when you get home the mail will still be delivered that you didn't delete while at work. It just allows copies to be sent instead of clearing the message off the server.

Sorry, forgot to identify them also.

IMAP - Internet Message Access Protocol
POP - Post Office Protocol

pippet · 07-22-2004, 11:24 PM

I have another question in my mind. A mail system can run with out an MDA, is that true?
If the user has configured the MUA to access local mail box, he can do it without an MDA , can't he?
i am telling u about KMail.
Also, if a mail is brought from the MTA through POP3, is it true that the message is deleted from the server?

pippet

motub · 07-29-2004, 05:41 AM

Quote:

Also, if a mail is brought from the MTA through POP3, is it true that the message is deleted from the server?

Not necessarily; under normal circumstances, when you download your POP3 mail, it is deleted from the server, but at least one mail client that I personally know of (Mozilla Thunderbird, therefore also Mozilla Mail and probably Netscape Mail), and probably others, allow you to check a setting (on Mozilla/Thunderbird, you can find this setting in the "Account Options" dialog), to "Leave messages on server". In that case, a copy of the message will be downloaded, and the message will remain on the server. Using that setting tends to screw up your new mail count in any notification utilities, as well as the user winding up with multiple copies of the same mail being downloaded to his local mail folders.

But I'm sure the setting is very useful under certain specific circumstances (like managing your mail when traveling for business or pleasure without losing the ability to keep a complete mail packet at home); just not so much for an ordinary home user downloading their mail.