This article will describes how to achieve a flexible and scalable email setup using opensmtpd and dovecot. For single-user or single-domain setups, this is an overkill, but feel free to read ahead, you may still find something useful.
I’ve used opensmtpd and dovecot for years now, and have been hosting email for several domains for a large portion of that time.
For small sites the text-based backends work fine, however, as the amount of users, domains and virtual users grows, it’s not easy to keep track. Data needs to be duplicated between smtpd’s credentials table, the virtual users table, and dovecot’s mailbox table.
I finally decided it was time to consolidate all the data in one place. I chose sqlite to begin with, but this can be moved onto postgresql if it needs to scale even more.
Opensmtpd and dovecot interaction
If you attemp to use virtualusers (and you’ll want to if you’re handling many
users in many domains), when receiving emails, opensmtpd maps email addresses
to usernames (which can contain no
@ sign). Dovecot then stores emails in
mailboxes based on these usernames. Both these things make mapping many virtual
users from different domains a bit compex.
I decided that the usernames I’ll be mapping to will take the form of
hugo_barrera.io). This makes a few initial settings a
bit complicated, but is infinitely flexible (the underscore sign in illegal for
email addresses, so there’s no change for collision).
I designed two database schemas, a normalized one, and a single-table one. I decided to keep the latter, since it makes inserts simpler and it’s easier to show-and-tell, but if you read through this entire article, you’ll be able to use whatever tableset you like.
Passwords are blowfish encrypted. Opensmtpd uses this by default and dovecot
also supports this (it refers to this scheme as
In different contexts, each column is used for something different:
- For email submission
passwordare used to validate authentication. The usernames smtpd receives from the client take the form of
- Opensmtpd needs to know what domains it’s receiving emails for. For this, it
just sees if there’s any entry in the
userstable where this domain is present. It would make no sense to keep a separate list of domains: if there’s no address for a domain, then I’re not accepting email for this domain.
- For email receiption,
username@domainis used to map to the recipient mailbox (ie: the
mailboxcolumn). Several addresses can map to a single user, and wildcards can also be used too. The
@is replaced with an
_when querying this table as well.
- For dovecot’s authentication, it needs to know if
username@domainis a real user, or just an aliased address. This is simply determined by checking if
password != NULL.
Passwords should be encrypted using either
doveadm pw -s BLF-CRYPT or
smtpctl encrypt. The output of both seems interchangeable.
DKIM signing is done with DKIMProxy. There’s a bunch of examples out there, so I won’t go into detail about that. Basically, opensmtp will send emails to DKIMProxy, accepts them back, tags them, and then relays them out.
First of all, we need to configure opensmtpd to receive email and read all the
data from the sqlite database. Here’s my
smtpd.conf as a reference:
This is all rather self-explanatory if you’re familiar with
sytax. I use lmtp via a unix socket because I believe it’s slightly faster, but
a network socket works fine too.
sqlite.conf is a bit more complex:
query_credentialsis used to validate user credentials. Opensmtpd will pass the provided email (in the form of username@domain), and this will find mapping rows. Since aliases have
password = NULL, those rows will return false.
query_domainis used when querying if we receive emails for a domain or not. The logic is rather simple: if there’s an address for a domain, then we accept email for it and viceversa.
query_userinfois used for email delivery, and to check if a mailbox exists. If there’s a mapping for it, then it exists.
username@domain, as described above.
On the smtpd side, that’s basically it. Here’s some sample data:
firstname.lastname@example.org an actual mailbox. It’ll get emails for himself, or anyone matching
%is the SQL wildcard character).
email@example.com another mailbox. That user will get email sent to him and just that.
If you’re going to have several overlapping delivery patterns, you probably
want to have a priority column in the table, and add
ORDER BY priority and
LIMIT 1 to some queries.
conf.d/auth-sql.conf.ext. I’ve modified it as follows:
mail_locationtell dovecot where relative to the user’s home the email is, and to use maildir. You can change this as you prefer.
override_fieldstells dovecot that email is always handled by the uid/gid
The other values are the defaults. Of course,
does require more changes so as to retrieve user data from the shared SQL
connectneeds to point to the same db that smtpd is using.
BLF-CRYPTsince that’s what smtpd uses.
password_queryis used to authenticate users (eg: those attempting to open their IMAP mailbox).
user_queryis used in two scenarios:
- To determine where to deliver messages destined to
user_domain. This is done by finding the real user who owns this mailbox (password must be null for aliased mailboxes!).
- To determine what messages to serve to a user reading his email (eg: via
IMAP). In this case, the usernames have the format of
- To determine where to deliver messages destined to
Setting the whole thing up is a bit complicated, but adding new users is a breeze. If there’s a need to grow, the sqlite db can become a postgresql db. By using lmtp, dovecot and opensmtpd can move into different machines, giving even more scalability. Further scaling, however, will require multiple dovecot backend and some changes to the sql schema.
Please feel free to point out any issues, potential improvements, or comments, I’ll try to update this appropiately.
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