Mail server clarifications

Scott Kitterman ubuntu at
Sat Nov 1 22:31:13 UTC 2008

On Sat, 1 Nov 2008 22:39:07 +0200 bvidinli <bvidinli at> wrote:
>There are generally three types of mail software, as i know:
>1- Mail clients : used to connect to mail servers, read meails, and
>send mails, with or without gui's.. example: evolution, thunderbird,
>outlook, sendmail client

These are also known as Mail User Agents (MUA).  The term client can be 
ambiguous because different applications in the mail system can, at 
different times be a client or a server.  This category also include pine, 
mutt, etc.  Any program that the user uses to read or write mail is an MUA.

>2- Mail Servers: the biggest part of picture, which actually sends and
>receives mails... handles/holds mails on servers...  also named as
>MTA, example: postfix, qmail, sendmail server

MTA (Mail Transfer Agent) is actually a bit narrower.  As the name implies, 
MTAs are responsible for moving mail from one place to another.  MTAs 
receive mail from a Mail Submission Agent (MSA), transfer it to another 
MTA, and after zero to a potentially large number of transfers (zero is 
local delivery and one is the most common external delivery scenario) 
transfers to a Mail Delivery Agent (MDA).  MDAs put the mail in the user's 
mail box.

>3- Mail connection agents: used to connect mail clients to mail
>servers, over network over pop3 and imap... these are for
>reading/getting/posting mails...not sending them actually to
>targets...... mail servers send email actually... example: dovecot,
>courier, cyrus
>there may be count a fourth type, which are mail delivery agents -
>MDA, in fact, i think these are part of mail servers...
>but if you use local mail, these handle mail locally...
>sendmail is a general term that is used for both a mail server and a
>separate mail client
>i dont know much about exim4 , but that may be a mixture of both mail
>server and type 2 mail connectors..
>what i suggest: postfix...
>it is easy to install and maintain, i think..
>as a hosting system, i suggest: ehcp, you may get that from
>it install everything needed for a hosting server: webserver, dns,
>mail, ftp, mysql and so on...
>i hope that would be usefull. if you have more question, you may ask me...

So generally the flow is MUA - MSA - MTA - ... - MTA - MDA - MUA.  In many 
cases these function are combined into a single application.

MUAs are generally specialized and just do MUA functions.

MSA is almost always combined with MTA in the same application.  

Most MTAs have some MDA capabilities, but dedicated MDAs with more 
capability are quite common.
>On Sat, Nov 1, 2008 at 10:15 PM, Tony Yarusso <tonyyarusso at> 
>> Hi, I've been reading various wiki pages and whatnot, but I'm still
>> having a little bit of trouble wrapping my head around the differences
>> between various mail server software.  I've been trying to figure out
>> things like MTA, MDA, etc., and it also seems like some packages are a
>> mix of both, so I'd just really appreciate it if someone could take
>> the time to explain in super-simple terms how everything relates
>> (either by mail or IRC - I'm in #ubuntu-server as tonyyarusso with an
>> always-connected screen session).  Generals would be a good start, and
>> in terms of any specifics things I'd be interested in knowing more
>> about include:
>> postfix

Primarily an MTA.  Full MSA support.  Limited MDA capability.

Ubuntu's recommended MTA.

>> exim4

Same general capabilities as Postfix AFAIK.

Default MTA in Debian and in Main in Ubuntu.

>> dovecot

Full featured MDA.  Supported (in Main) MDA for Ubuntu.

>> courier

MSA/MTA/MDA.  Does it all.  Separate packages for different functions so you could, for example 
use Courier as an MDA with Postfix as your MTA.

>> procmail

MDA.  Not easy to use.

>> cyrus

Complete mail system like Courier.  The MDA has a reputation for being fast 
and good for large deployments, but complex to set up.

>> qmail

Ancient MTA.  Some people swear by it still, but IMO its day has passed.

Sendmail - The original MTA.  Still alive and kicking.  It's reputation for 
have opaque and complex config files is well deserved.  Also provides 
libmilter for Milter plugins.  This library can also be used by Postfix.

Also used as a generic term.  Several MTA packages provide a 'sendmail' 

Scott K

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