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-   -   What is Sendmail and Exim (

vikasumit 05-15-2006 04:54 AM

What is Sendmail and Exim

I have been reading about the two terms/software

Sendmail and Exim,

and really got confuse what they are. for me sendmail is a mailing server, and Exim .... not sure what it is.

Qmail is Mail server and you either need Sendmail or Qmail install in order to make a mail server, but not sure if Exim is mail server or something similar.

I am really confuse, and also I need a way that help me understanding these thinks better, and Like If I have to configure mail server on a computer that host 3-4 domains for Web hosting than how can I do taht..

Please help me

timmeke 05-15-2006 05:31 AM

Sendmail and QMail, but also others like Postfix (a sendmail look-alike), are called MTAs (Mail Transport Agents). They are designed, as their name suggest, to transport mails over a network.
In the case of sendmail, you can also use it directly from the command line to send out mails, as if it was a MUA (Mail User Agent).

Basically, a MUA is a piece of software that acts as a mail client. It allows you to read mails and send them out. Most MUAs will simply initiate a connection to an MTA for sending out the mails. The MTA is considered responsible for the correct routing of the mail (and for performing some mail filtering).

On most hosts, an MTA like QMail or Sendmail is running and listening on port 25 (SMTP - Simple Mail Transfer Protocol). Any program that connects to this port and then issues commands following the SMTP protocol, will send out mails via the MTA. This is handy for sending out mails. Those MTAs are only listening to port 25 on the "localhost" loopback network interface (by default, for security reasons), so they cannot be used from other hosts than the one they are running on.

A "mail server", as you call it, however, is defined as a machine that:
-accepts incoming mails (again, port 25) from other machines
-allows the retrieval of mails that were received (ie via POP3 or IMAP protocols typically).
Since most MTAs on simple hosts do not accept mails from other machines, they are not to be considered as "complete mail servers".

Most MUAs know how to contact mail servers via SMTP to send out mails and via POP3/IMAP to retrieve mails.

Personnally, I don't know Exim, but on the Exim home page - -, the authors of Exim claim
that it is an MTA, just like sendmail and QMail.

vikasumit 05-15-2006 06:45 AM


Thanks for your detail explanation of MTA and MUA. Obviously they arose some more question in my mind.
So, If I take Exim as MTA, than I can either have Exim or Sendmail or Qmail on my server, as port 25 is taken by one application only.

Actually some one confuse me on Exim been a MTA only :)

but it seems it is.

Thanks for your help, if anyone else as any info please share. I am doing a Project on email and don't know how to start with it. My project is like Email Management Software, where I need to configure email account on server for different virtual hosts of apache Web server.

timmeke 05-15-2006 07:40 AM

Indeed, it is possible to install multiple MTAs on the same host, since only 1 application can
listen on port 25 of a given network interface.

However, it is often not advisable. I'll explain.
Historically, sendmail was probably the first. After that, a few look-alikes, like Postfix, were developed, because sendmail can be very tricky to configure (it can involve m4 macros and stuff like that).

Because Postfix is a "drop in" replacement for sendmail (it actually replaces the binaries of sendmail with symlinks to it's own), installing both postfix and sendmail on the same machine is a bad idea.
The same may hold for other MTAs as well.

Think also about the config files shared by those MTAs, like the /etc/aliases file for instance. If you decide to install multiple MTAs together on one host, you should keep this in mind too, to avoid conflicts.

Theoretically, you could install 1 MTA for each network interface you have (since each of the network interfaces has it's own port 25) - ie one for your public IP interface and one on localhost interface -. But this is also "not done", to avoid configuration issues like the ones 1 mentioned above.

vikasumit 05-15-2006 07:56 AM


I was looking at My Local server which has Cpanel install on it. and as I told my application resemble in some sense to cpanel's Mail Working, it can help me as well.

anyways, there I found on PC that I have cpanel demo install it has both Exim and sendmail binaries, though I cannot figure out actually which one is working..

When I do
#sendmail -i
it gives Exim information , that is, Exim is MTA and blah blah...

and when I do
same information is shown

Anyone Else know about Exim and Sendmail please let me know...

timmeke your knowledge is great about the stuff. can you please share more of your knowledge on this ..


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