Setting up a local dev mail server August 5, 2018 on Drew DeVault's blog

As part of my work on lists.sr.ht, it was necessary for me to configure a self-contained mail system on localhost that I could test with. I hope that others will go through a similar process in the future when they set up the code for hacking on locally or when working on other email related software, so here’s a guide on how you can set it up.

There are lots of things you can set up on a mail server, like virtual mail accounts backed by a relational database, IMAP access, spam filtering, and so on. We’re not going to do any of that in this article - we’re just interested in something we can test our email code with. To start, install your distribution of postfix and pop open that /etc/postfix/main.cf file.

Let’s quickly touch on the less interesting config keys to change. If you want the details about how these work, consult the postfix manual.

• myhostname should be your local hostname
• mydomain should also be your local hostname
• mydestination should be $myhostname, localhost.$mydomain, localhost
• mynetworks should be 127.0.0.0/8
• home_mailbox should be Maildir/

Also ensure your hostname is set up right in /etc/hosts, something like this:

127.0.0.1 homura.localdomain homura

Okay, those are the easy ones. That just makes it so that your mail server oversees mail delivery for the 127.0.0.0/8 network (localhost) and delivers mail to local Unix user mailboxes. It will store incoming email in each user’s home directory at ~/Maildir, and will deliver email to other Unix users. Let’s set up an email client for reading these emails with. Here’s my development mutt config:

set edit_headers=yes
set realname="Drew DeVault"
set from="sircmpwn@homura"
set editor=vim
set spoolfile="~/Maildir/"
set folder="~/Maildir/"
set timeout=5
color index blue default ~P

Make any necessary edits. If you use mutt to read your normal mail, I suggest also setting up an alias which runs mutt -C path/to/dev/config. Now, you should be able to send an email to yourself or other Unix accounts with mutt¹. Hooray!

To accept email over SMTP, mozy on over to /etc/postfix/master.cf and uncomment the submission service. You’re looking for something like this:

127.0.0.1:submission inet n       -       n       -       -       smtpd
#  -o syslog_name=postfix/submission
#  -o smtpd_tls_security_level=encrypt
#  -o smtpd_sasl_auth_enable=yes
#  -o smtpd_tls_auth_only=yes
#  -o smtpd_reject_unlisted_recipient=no
#  -o smtpd_client_restrictions=$mua_client_restrictions
#  -o smtpd_helo_restrictions=$mua_helo_restrictions
#  -o smtpd_sender_restrictions=$mua_sender_restrictions
#  -o smtpd_recipient_restrictions=
#  -o smtpd_relay_restrictions=
#  -o milter_macro_daemon_name=ORIGINATING

This will permit delivery via localhost on the submission port (587) to anyone whose hostname is in $mydestination. A good old postfix reload later and you should be able to send yourself an email with SMTP:

$ telnet 127.0.0.1 587
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
220 homura ESMTP Postfix
EHLO example.org
250-homura
250-PIPELINING
250-SIZE 10240000
250-VRFY
250-ETRN
250-ENHANCEDSTATUSCODES
250-8BITMIME
250-DSN
250 SMTPUTF8
MAIL FROM:<sircmpwn@homura>
250 2.1.0 Ok
RCPT TO:<sircmpwn@homura> 
250 2.1.5 Ok
DATA
354 End data with <CR><LF>.<CR><LF>
From: Drew DeVault <sircmpwn@homura>
To: Drew DeVault <sircmpwn@homura>
Subject: Hello world

Hey there 
.
250 2.0.0 Ok: queued as 8267416366B
QUIT
221 2.0.0 Bye
Connection closed by foreign host.

Pull up mutt again to read this. Any software which will be sending out mail and speaks SMTP (for example, sr.ht) can be configured now. Last step is to set up LTMP delivery to lists.sr.ht or any other software you want to process incoming emails. I want most mail to deliver normally - I only want LTMP configured for my lists.sr.ht test domain. I’ll set up some transport maps for this purpose. In main.cf:

local_transport = local:$myhostname
transport_maps = lmdb:/etc/postfix/transport

Then I’ll edit /etc/postfix/transport and add these lines:

lists.homura lmtp:unix:/tmp/lists.sr.ht-lmtp.sock
homura local:homura

This will deliver mail normally to $user@homura (my hostname), but will forward mail sent to $user@lists.homura to the Unix socket where the lists.sr.ht LMTP server lives.

Add the subdomain to /etc/hosts:

127.0.0.1 lists.homura.localdomain lists.homura

Run postmap /etc/postfix/transport and postfix reload and you’re good to go. If you have the lists.sr.ht daemon working, send some emails to ~someone/example-list@lists.$hostname and you should see them get picked up.