Minimal MTA that delivers mail locally (for cron)?

Question

On my laptop, I have set up a cron that performs a weekly job. If there is something to report, I would like to get an e-mail for it. Right now I am using KMail as MUA which is able to read mail from the Maildir directory at ~/.local/share/local-mail. Thus, I want local mail to be delivered to that folder (with a Maildir structure).

I was hoping that a dead simple program already exist that has a sendmail interface (such that echo "$REPORT" | mail -s "$SUBJECT" "$ME" can work with it). Installing exim or forwarding mail to my remote mailserver is considered overkill.

The question Simplest way of forwarding all mail from server? seems to target remote forwarding which does not suit my needs (I need local delivery). This old Gentoo thread ended up in crafting an old script in Perl. Surely there must exist a well-thought, dead-simple program?

Any recommendations? I am using Arch Linux.

Answer 1

Since I could not find an existing, small program, I decided to write my own one. Originally, I came up with:

#!/bin/bash
# Simple sendmail
# filename per spec at http://cr.yp.to/proto/maildir.html
rand=$((RANDOM % 1000))
msgname=$(date +%s).P$$R$rand.$(hostname | tr '/:' '\057\072')

# Safety measure: do not overwrite existing mail
set -o noclobber

cat > ~peter/".local/share/local-mail/inbox/new/$msgname"

That worked... except if the user executing the script is not "peter". The successor of this idea is a "small" C program, femtomail. From its README:

femtomail - minimal MDA with Maildir support

femtomail is a minimal Mail Delivery Agent (MDA) for local mail. Mail is accepted from standard input and placed in a Maildir box of a user. This software is intended for use on a single-user machine.

Remote delivery, daemonizing, sender verification, etc. is not implemented and won't be implemented due to its complexity. femtomail is not written because mail software did not exist, but because existing software were too large for the simple task of delivering cron mail to the local user.

The workflow of femtomail:

1. Change the process user and group.
2. Create a new file with a unique filename in the mail directory.
3. Write a Received header to the file.
4. Pass data from standard input to the file.
5. Exit.

The source code and installation instructions are available at https://git.lekensteyn.nl/femtomail/.

Answer 2

Based on the accepted answer, I've created a small solution using the Docker image form the official Dovecot CE repository at https://hub.docker.com/r/dovecot/dovecot, which I'd like to share.

1. Run the container:

   docker run --name dovecot -d dovecot/dovecot
   

2. Install sudo, which isn't included in the dovecot image, but needed for our script

   apt-get update && apt-get -y install sudo
   

3. Set up a mail directory. I found it to be more flexible and less hassle to do so in /var/mail rather than in the user home directory:

    mkdir /var/mail/vmail
    chown vmail /var/mail/vmail  
   

4. Create a mailbox user, who must be member of the vmail group:

    useradd -g vmail theresa
   

   You can add more users anytime if need be.

5. Tell Dovecot where the mail is. To do so, we need to change mail_location in /etc/dovecot/dovecot.conf to this:

    mail_location=maildir:/var/mail/vmail/%u
   

   You need to restart dovecot afterwards, which can be done by simply restarting the Docker container:

    docker restart dovecot
   

6. Create the following bash script at /usr/local/storemail.sh in the dovecot container:

    #!/bin/bash
   
    # Store mail in maildir
    # Based on https://unix.stackexchange.com/a/82367/215688
   
    # Simply stores an email to the user mail directory 
   
    USER=$1
   
    MAIL_LOCATION="/var/mail/vmail/$USER"
    # filename per spec at http://cr.yp.to/proto/maildir.html
    rand=$((RANDOM % 1000))
    msgname=$(date +%s).P$$R$rand.$(hostname | tr '/:' '\057\072')
   
    # Just in case the user dir has not been created yet
    sudo -u vmail mkdir -p "$MAIL_LOCATION/new"
   
    # Safety measure: do not overwrite existing mail
    set -o noclobber
   
    # now store the email there
    sudo -u vmail cat > "$MAIL_LOCATION/new/$msgname"
   

   Don't forget to make it executable:

    chmod +x /usr/local/storemail.sh
   

That's it.

Now you can store a new mail using the script. The only argument is the username, and the email needs to be provided from stdin.

    storemail.sh theresa <testmail.eml

Or from outside the container use

    docker exec -i dovecot <testmail.eml storemail.sh theresa

Or even from a remote machine via ssh

    ssh docker-machine.local <testmail.eml docker exec -i dovecot storemail.sh theresa
    

I recommend to create a custom dockerfile that automates the preparation steps.