---
title: Types of email domains
lastUpdated: 2023-09-25
slug: ispmail-bookworm/types-of-email-domains
sidebar:
  order: 50
---

import { Aside } from "@astrojs/starlight/components";

This is the pretty boring but at the same time very important part of the tutorial. Do not skip it. Most problems that
readers have with their mail servers are caused by a misunderstanding of the different types of email domains. There is
nothing you need to do on your server right now. Just lean back and relax and make sure that you read and fully
understand this page. It will spare you frustration.

As explained on the previous page Postfix is the software component that speaks SMTP. It receives emails from other
server and sends email out. Of course you will only want to receive emails for your domains only.

Postfix will accept emails for any of these [classes of domains](http://www.postfix.org/ADDRESS_CLASS_README.html):

1. Canonical domains (also called _local domains_)
2. Hosted domains
   - Virtual mailbox domains
   - Virtual alias domains
3. Relay domains

Let us look at the first two in detail. The third one – relay domains – is not needed in this context.

## Canonical domains

Once upon a time users logged into servers using [Telnet](https://en.wikipedia.org/wiki/Telnet) or
[SSH](https://en.wikipedia.org/wiki/Secure_Shell) to read their email in basic text-based mail clients. Their email
addresses were often just a combination of their login name and the server name. Something like
`mpauls@server17.biology.example.net`.

At least that made things pretty simple for the server administrator. He just accepted emails for any valid login user
on that server. The mail server knew what to do. It checked if “mpauls” is found in /etc/passwd and then stored emails
in “/var/mail/mpauls”.

If you wanted to do it that way…

1. You would need to create system accounts for all users who want to receive email. Every system user is a potential
   security risk because they can login to the server using a shell. If the user chooses a weak password you are
   essentially inviting attackers to harm your server.
2. If you want to host many email accounts this becomes impractical.
3. Postfix cannot distinguish the local domains. If you have three local domains “example.org”, “example.com” and
   “example.net” then all these email addresses would lead to the same mailbox: `john@example.org`, `john@example.com`
   and `john@example.net`. So you cannot use different domains for different purposes.

To tell Postfix which domain you consider local you list them in the “mydestination” configuration setting in your
main.cf configuration file. Example:

```
mydestination = example.org, example.com, example.net
```

<Aside type="caution">
  A domain never belongs to more than one class. You would get a warning in your log file and Postfix will start
  behaving strangely. One domain can only have one purpose. This is a frequent mistake.
</Aside>

It is common however to use the server’s hostname as a local domain. If your server is called “scully.example.com” you
could set “mydestination = scully.example.com”. Or you just set it to “mydestination = localhost” if you rather want to
use “scully.example.com” as a virtual domain. Some parts of your system may send emails to root@localhost so this is a
sane setting.

## Virtual domains

This is the class of domains that will be our workhorse. Postfix allows us to add an unlimited number of domains that
can receive emails. That’s what an ISP does and so will we.

Where do we put the list of domains? Postfix is pretty flexible. We could put it in a huge text file. We might use LDAP
if we have some kind of user directory in our organization. But in this guide we will store that information in a simple
MariaDB database.

Postfix handles hosted email addresses by checking two _mappings_:

- Virtual Aliases (“redirect this email address to another address”)
- Virtual Mailboxes (“accept email for this email address”)

So much terminology. Time for an example, isn’t it? Let’s tell Postfix we want to have three email addresses:

- `john@example.org`
- `jack@example.org`
- `jack@example.com` _(note that this is another domain than example.org)_

Postfix only understands _mappings_. They are much easier than they sound. A mapping is just a table that has a left and
a right column. Postfix always looks for stuff in the left column (_key_) to find more information in the right column
(_value_). Like this:

| Virtual mailbox (key) | Virtual mailbox location on disk (value) |
| --------------------- | ---------------------------------------- |
| `john@example.org`    | /var/vmail/example.org/john/Maildir      |
| `jack@example.org`    | /var/vmail/example.org/jack/Maildir      |
| `jack@example.com`    | /var/vmail/example.com/jack/Maildir      |

So the left column lists the valid email addresses. And the right column is apparently some path on disk. Right, that’s
all the magic. If an email for `jack@example.org` is received then Postfix will find the entry in the left column and
can figure out where Jack’s mails are stored on disk.

Before checking if a specific email address is valid Postfix first checks if it is responsible for the domain at all.
That’s done by this mapping:

| Virtualdomain (key) | Whateve (value) |
| ------------------- | --------------- |
| example.org         | Kittens         |
| example.com         | Puppies         |

So there are two domains in the left column. But what is that in the right column? Kittens? Really? Well, the truth is:
it doesn’t matter. It can be anything. So why is that column there at all? The reason is that _mappings_ always have two
columns. Postfix wants a list of domains but it has no concepts for one-dimensional _lists_. So it uses this format and
ignores the right column.

Essentially these two mappings are all we need. Add a little configuration and you can already receive emails. However
Postfix provides another useful feature: _aliases_. An alias is a redirection (or _forwarding_) of one email address to
one or more other addresses. Possible uses:

- forward postmaster@… for all your domains to one mailbox
- create an email distribution list for a team or department
- forward copies of all your emails to another address
- redirect emails meant for a coworker who has left the organisation

As usual, Postfix expects a mapping for aliases. Time for another example:

| Step | Virtual email address (left) | Redirect to (right)                       |
| :--- | :--------------------------- | :---------------------------------------- |
| 1    | `postmaster@example.org`     | `jack@example.com`                        |
| 2    | `abuse@example.org`          | `jack@example.com`                        |
| 3    | `jack@example.com`           | `jack@example.com`,`jacky@workaround.org` |
| 4    | `sophie@example.net`         | `hr@example.net`                          |
| 5    | `hr@example.net`             | `rick@example.net`                        |
|      | `hr@example.net`             | `tina@example.net`                        |
| 6    | `@example.org`               | `jack@example.com` (DANGEROUS)            |

Quite a lot happens here. How does Postfix interpret this table?

1.  Redirect emails for `postmaster@example.org` to `jack@example.com`
2.  Redirect emails for `abuse@example.org` also to `jack@example.com`
3.  Keep a copy of an incoming email for `jack@example.com` in his mailbox and send another copy to
    `jacky@workaround.org.` Yes, you can use multiple email addresses seperated by commas.
4.  Sophie has left the organisation so her address is forwarded to HR.
5.  The HR team consists of two members: `rick@example.net` and `tina@example.net.` Every email to `hr@example.net` will
    be forwarded to each of them. This works similar to (3) but this time the target addresses are not seperated by
    commas but stored in different rows.
6.  Forward any email address of the _example.org_ domain to `jack@example.com.` We call this a _catch-all_ address.
    This is dangerous for two reasons. First it will send the poor Jack anything that is sent to the _example.org_
    domain if there is no user account for that address. (Yes, spammers guess addresses.) And second it disables a
    security check in Postfix that the destination address `jack@example.com` actually exists. Postfix may first receive
    the email and then be confused if the recipient is not reachable. That leads to backscatter and harms your
    reputation. But don’t worry. I will show you the proper way to use catch-all addresses later.

So basically this is the way that Postfix handles aliases:

1.  Is the domain of the email address defined as a virtual domain?
2.  Get all rows of the table where the email address is found in the left column. Send a copy of the email to everyone
    mentioned in the right column. If the right column contains multiple email addresses separated by commas then split
    them first.
3.  If there was no such row then check again if there is an `@domain` row.
4.  Still nothing found? Then reject the email.

<Aside type="tip">
  Two email addresses are mandatory for every domain that you host. `postmaster@domain` and `abuse@domain`. These
  requirements are documented in RFC 521 and RFC 2142. Be sure to add aliases for them. If an email to those addresses
  would bounce, your domain reputation will suffer.
</Aside>

## Database

Did you get the idea of mappings? Two columns in a table? Keys and values? Good. Now how do those mappings work when we
want to put the information into an SQL database?

Basically a relational database works in rows and columns, too. So we can take the format of the tables shown above and
put them into database tables. Let’s call these tables…

- virtual_domains (for the list of domains)
- virtual_aliases (for the aliases mapping)
- virtual_users (for the email users and their mailboxes respectively)

I won’t bore you with the SQL stuff right now. We will deal with that in a later chapter. Let’s just briefly cover how
Postfix can get data from the database. We provide Postfix with _configuration files_ for that purpose. These files
often have a “.cf” suffix (**c**onfiguration **f**ile).

Let’s take the virtual_users table for example. This is the content of a file that is located at
`/etc/postfix/mysql-virtual-mailbox-maps.cf`:

```
# Information on how to connect to your MySQL server
user = someone
password = some_password
hosts = 127.0.0.1
dbname = mailserver

# The SQL query string
query = SELECT mailbox_path FROM virtual_users WHERE email_address='%s'
```

The first four lines describe how the database can be connected to. You provide a database user and database password as
well as the IP address or hostname of the SQL server and the name of the database.

A more interesting bit is the _query_ that is defined in the last line. Postfix still thinks in left and right columns.
So it will run this query and replace the %s by the email address it is looking for – that’s what is expected in the
left column. The database will then get all rows from the database table that match this criterion. If one row matches
the query then the “SELECT mailbox_path” will return just the value of what Postfix would expect to be in the right
column.

That’s all the magic that Postfix needs to talk to your SQL database. You tell Postfix how to connect to the database
and how the data in the database table corresponds to the left and right columns.

Note that a lookup here must only return just one row from the database. Postfix must uniquely know where the mailbox
path for a given user is. There are other mappings though where it’s allowed to have multiple right-hand side items for
one left-hand side item – for example in virtual aliases.

To use the above configuration file you have to configure it in Postfix’s main.cf file:

```
virtual_mailbox_maps = mysql:/etc/postfix/mysql-virtual_mailbox_maps.cf
```

If you find that this mapping is not working as you expected then the “postmap -q” command is your friend. You can ask
Postfix what the right-hand side value for a given left-side value is. Say that you are interested in the mailbox_path
for the email_address `john@example.org`:

```
postmap -q john@example.org mysql:/etc/postfix/mysql-virtual_mailbox_maps.cf
```

Postfix will then run the above SQL query with your “`john@example.org`” argument:

```
SELECT mailbox_path FROM virtual_users WHERE email_address='john@example.org'
```

The result should be:

```
/var/vmail/example.org/john/Maildir
```

In this guide we will use a slight variation of that SQL query:

```
query = SELECT 1 FROM virtual_users WHERE email='%s'
```

Postfix does not need to know where the mailbox is located on disk. We will not use Postfix to store the email to disk.
Instead we will make Postfix hand over the email to Dovecot which stores the email. Dovecot has a couple of nice extra
features (like server-side scripts) that we want to use. So Postfix just needs to know whether an email address is valid
or not. We just return “1” because actually the right column is not considered anyway. Postfix just needs to know
whether there is a database row or not.
