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|
-----
ESMTP
-----
José Fonseca
---
July 2003
Introduction
~~~~~~~~~~~~
<<esmtp>> is a user configurable relay-only Mail Transfer Agent (MTA) with a
<<sendmail>> compatible syntax. It's based on <<libESMTP>> supporting the
AUTH (including the CRAM-MD5 and NTLM SASL mechanisms) and the StartTLS SMTP
extensions.
See the {{{http://esmtp.sourceforge.net/}ESMTP home page}} for updated
information.
Rationale
~~~~~~~~~
Since the moment I switched to Linux I had some problems to have email
properly configurated to my personal needs. I use a POP3/SMTP mail account
for my personal mail, and I usually connect to the internet with my laptop
via several LANs (home, work, ...) with several different firewall
configurations. The standard MTAs didn't provided the flexibility I needed --
they either failed to deliver the mail directly or failed to authenticate
with the relay, or were configured system-wide and conflicted with the local
MTA.
<<fetchmail>> always worked fine to get email, but I needed a <<sendmail>>
alike program to send email from <<Mutt>>. Basically I wanted a program
which could be for SMTP what <<fetchmail>> was for POP3, i.e., an user
configurable MTA with authorization support.
Some small modifications to the <<<mail-file>>> example from <<libESMTP>> did
the trick. After receiving positive feedback from the <<libESMTP>> author,
Brian Stafford, I decided to make the program more confortable for others to
use -- adding a command-line option parser extracted from sendmail, a
configuration file parser from <<fetchmail>>, a man page based on <<ssmtp>>
and <<sendmail>> man pages and using autotools to wrap it all up. The result
of this cut and paste effort is <<esmtp>>. Well, the first version, that is.
Since then a few more bells and whistles were added, with the help of some
contributors.
Configuration
~~~~~~~~~~~~~
* Sample configuration file
~~~~~~~~~~~~~~~~~~~~~~~~~~~
This is a simple configuration file for a quick start:
--------------------------------------
hostname = mail.myisp.com:25
username = "myusername"
password = "mysecret"
starttls = enabled
identity = myself@somewhere.com
hostname = smtp.somewhere.com:25
username = "myself"
password = "secret"
starttls = enabled
mda "/usr/bin/procmail -d %T"
--------------------------------------
* Configuration options
~~~~~~~~~~~~~~~~~~~~~~~
Options are speficied by giving a <keyword=value> in the configuration
file. The equal sign is optional and can be replaced by whitespace. The
value may be enclose in simple or double quotes, in which case special
characters can be escaped as in normal <C> strings.
[hostname] - set SMTP host and service (port).
This is specified in the format <<<host.example.org[:service]>>> with no
whitespace surrounding the colon if service is specified. service may be a
name from <<</etc/services>>> or a decimal port number. If not specified
the port defaults to 587.
Note (from <<libESMTP>> documentation): the default port number is set to
587 since this is the port that should be used for mail submission, see RFC
2476. By choosing this default now, the API does not change behaviour
unexpectedly in the future as use of the new standard becomes commonplace.
The hostport notation simplifies things for the application, the user can
type <<<localhost:smtp>>> or <<<localhost:25>>> where the application
expects a host name.
[username] - set the user name.
[password] - set the password.
[starttls] - determine the usage of the StartTLS extension.
It can be one of <"enabled">, <"disabled"> or <"required">. It defaults to
disabled.
[certificate_passphrase] - set the certificate passphrase.
[identity] - define an identity.
An identities is a set of options associated with a given address. For
example:
--------------------------------------
identity = myself@somewhere.com
hostname = smtp.somewhere.com:25
username = "myself"
password = "secret"
--------------------------------------
Identities are be selected by the address specified in the <-f> flag. You
can have as many you like.
The options up to the first <identity> option constitute the default
identity.
Note: the default identity settings are not shared by the other identities.
Everything (username, password, etc.) must be specified for every identity
even if they don't differ from the default identity.
[mda] - set the Mail Delivery Agent (MDA).
Local delivery addresses will be inserted into the MDA command wherever you
place a <%T>. The mail message's From address will be inserted where you
place an <%F>.
Some common MDAs are <"/usr/bin/procmail -d %T">,
<"/usr/bin/deliver"> and <"/usr/lib/mail.local %T">.
See below for more information about using <<esmtp>> with an MDA.
Invocation
~~~~~~~~~~
<<esmtp>> is command line compatible with <<sendmail>>.
See the <<esmtp>> man page for information on how to invoke it.
Interfacing to Mail User Agents
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Most Mail User Agents (MUAs) will work without need to configuration provided
that you install a symbolic from <<</usr/sbin/sendmail>>> to the esmtp
executable. This should already be taken care of by the <install> target of
the makefile when building from source.
If by any reason it is not possible to have (such as no administrator
priviledges or the use of another MTA for local delivery) then you will have
to reconfigure your MUA to use the esmtp executable instead.
* Mutt
~~~~~~
If not using a symbolic link to the esmtp executable you can make <<Mutt>> use
<<esmtp>> by adding the following line to your <<<~/.muttrc>>>:
-----------------------------
set sendmail="/path/to/esmtp"
-----------------------------
<<Esmtp>> supports <<sendmail>> envelope sender <-f> flag, and you are
advised to always enable it by adding the following line to <<Mutt>>
configuration file:
---------------------
set envelope_from=yes
---------------------
For debugging purposes you may prefer to put in your <<<~/.muttrc>>>:
--------------------------------------------------
set sendmail="/path/to/esmtp -v -X /tmp/esmtp.log"
--------------------------------------------------
This will enable verbose output and logging of the traffic with the SMTP
server.
Interfacing to Mail Delivery Agents
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<<esmtp>> relies upon a Mail Delivery Agent (MDA) for local mail delivery, so
you need one if you want to avoid having another MTA for local delivery.
Notice that at the moment <<esmtp>> does not honor mail aliases or
<<<.forward>>> files.
To deliver to other users beside yourself, the MDA must be installed with
<setuid> flag -- which is done by default in most Linux distributions.
* Procmail
~~~~~~~~~~
To use <<procmail>> with <<esmtp>> set the <<<mda>>> configuration value to:
-----------------------------
mda="/usr/bin/procmail -d %T"
-----------------------------
Interfacing with other mail applications
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Fetchmail
~~~~~~~~~~~
By default <<fetchmail>> delivers messages via SMTP to port 25 on the machine
it is running. Because <<esmtp>> has no SMTP server if you are not using another
MTA for local delivery then you will need to configure <<fetchmail>> to use
<<esmtp>> executable. This is acomplished by adding the following lines to the top
of your <<<~/.fetchmailrc>>>:
-------------------------------------
defaults
mda "/path/to/esmtp -f %F %T"
-------------------------------------
Since <<esmtp>> simply forwards the mail to another MDA you can avoid this
redundant step by simply replacing the value inside the quotes above by
whichever value you use on your <<<~/.esmtprc>>>.
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