---
title: Quotas
lastUpdated: 2023-10-04
slug: ispmail-bookworm/quotas
sidebar:
  order: 140
---

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

<Aside type="tip" title="Optional feature">
  This feature is completely optional. If you are eager to get finished then skip this page and maybe come back later.
</Aside>

Quotas are size limits for users. You can make sure that users do not waste arbitrary amounts of disk space but are
forced to clean up old emails every now and then.

The magic happens in two places:

1. Postfix needs to reject new emails if the user’s mailbox is over quota.
2. Dovecot needs to keep track of the quota and how much the user has already used up of it.

### Dovecot quota policy service

Let’s start with Dovecot. Find the file `/etc/dovecot/conf.d/90-quota.conf` and edit it. There are several `plugin {}`
sections. Take one and make it look like:

```
plugin {
  quota = count:User quota
  quota_vsizes = yes

  quota_status_success = DUNNO
  quota_status_nouser = DUNNO
  quota_status_overquota = "452 4.2.2 Mailbox is full and cannot receive any more emails"
}
```

The first line defines that you want to calculate the used space in a user’s _maildir_. There are several
[backends](https://doc.dovecot.org/configuration_manual/quota_plugin/) like that but the
_[count](https://doc.dovecot.org/configuration_manual/quota/quota_count/#quota-backend-count)_ is the best choice in
this context. (Previous guides used _maildir_ here.) The string “User quota” is just an arbitrary string that may be
queried from a mail user agent.

The lines starting with “`quota_status_…`” set return values for the service that you will set up in a minute. It will
tell Postfix that it will not interfere (_DUNNO_ – colloquial way to say “I don’t know”). And it will return a string
with a return code 452 if the user is over quota. Codes starting with “4” mean temporary errors. It will tell the
sending party that it is worth retrying at a later time. However if the user does not resolve the issue it will lead to
a _bounce_ error email after three days.

In the same file (_90-quota.conf_) add another section:

```
service quota-status {
  executable = /usr/lib/dovecot/quota-status -p postfix
  unix_listener /var/spool/postfix/private/quota-status {
    user = postfix
  }
}
```

This creates a new [Dovecot service](https://doc.dovecot.org/configuration_manual/service_configuration/) responding to
requests from other processes. You surely recognize that we put it into the jail that Postfix runs in
(_/var/spool/postfix_), so that Postfix can access it.

Time to restart Dovecot:

```
systemctl restart dovecot
```

Take a look at the /var/spool/postfix/private directory. If all went as intended you will find a socket file called
`quota-status` there. Otherwise please check the `/var/log/mail.log` file for errors.

### Postfix recipient restrictions

If we stopped here, then Dovecot would reject emails for users who have no space left. However Postfix would still
happily receive new emails and attempt to forward them to Dovecot via LMTP. Dovecot however will deny that. It will then
keep the email in its queue and retry for a while. In the end it will send a _bounce_ back to the sender telling them
about the problem. So why is this bad?

1. The sender will assume that the email was delivered while it is stuck in the queue for up to three days.
2. Spam emails use forged senders. So at the time that Postfix generates the _bounce email_ it will likely send it to an
   innocent person. This is called _backscatter_ and considered a mail server misconfiguration. Such a problem may get
   your mail server blacklisted. You don’t want that.

So the next logical step is to make Postfix check whether a mailbox is over quota whenever a new email arrives. Let’s
hook up into the “RCPT TO” phase of the SMTP dialog when a new email comes in. Postfix checks its
_smtpd_recipient_restrictions_ configuration at this stage. Run this command in the shell:

```
postconf smtpd_recipient_restrictions=reject_unauth_destination, \
    "check_policy_service unix:private/quota-status"
```

This adds two checks:

1. `reject_unauth_destination` checks whether the mail server is the final destination for the recipient’s email
   address. This is pretty much the default behavior if you do not define any restrictions.
2. `check_policy_service` connects to the socket file at `/var/spool/postfix/private/quota-status` that was put there by
   Dovecot. It will use it to ask Dovecot whether the user is over quota in which case the email would get rejected.

### Test it

If you are curious to see this working, then set John’s mailbox quota to 5 KB:

```sql
# mariadb mailserver
mysql> update virtual_users set quota=4000 where email='john@example.org';
```

Send him a few emails using the ‘swaks’ tool:

```
swaks --server localhost --to john@example.org
```

After a few emails you will see the rejection message:

```
-> RCPT TO:john@example.org
 <** 452 4.2.2 john@example.org: Recipient address rejected: Mailbox is full and cannot receive any more emails
```

### Troubleshooting

These are things you should consider if quotas do not seem to work properly:

- Check if you have enabled “quota” in the “mail_plugins” in the 10-mail.conf file.
- Your users may complain that they have deleted many emails but are still over quota. Let them check if they actually
  emptied the _Trash_ folder. Of course emails in that folder also contribute to the disk space usage. Once the Trash
  folder is expunged the problem should be gone. You may also allow your users more space in the Trash folder. That’s
  explained in the [Dovecot documentation](https://doc.dovecot.org/configuration_manual/quota/#quota-rules).
- If you directly remove files from a user’s Maildir instead of properly accessing the mailbox using IMAP then you will
  screw up the quota calculation. In that case let Dovecot recalculate the quota:  
   `doveadm quota recalc -u john@example.org`

### Automatic warning emails

The last step is to inform the poor users if they accidentally went over quota. After all they do not necessarily
recognize that on their own. Let’s do that by sending them an email with a warning. Yes, we will make sure that the
email gets through even if the quota is reached.

Edit the `90-quota.conf` file again. Add this section to the file (derived from the
[Dovecot documentation](https://doc.dovecot.org/configuration_manual/quota/#quota-warning-scripts)):

```
plugin {
   quota_warning = storage=95%% quota-warning 95 %u
   quota_warning2 = storage=80%% quota-warning 80 %u
}
service quota-warning {
   executable = script /usr/local/bin/quota-warning.sh
   unix_listener quota-warning {
     user = vmail
     group = vmail
     mode = 0660
   }
}
```

This section defines two automatic quota warnings. The first (quota_warning) is triggered if the user reaches 95% of the
quota. The second (quota_warning2) at 80%. These lines follow this schema:

- **Trigger** (e.g. “storage=95%”). The “%” sign needs to be used twice if you want to emit a literal percent sign. So
  this is not a typo.
- The **socket** you want to call in that case. Our socket is the “service quota-warning” that calls a shell script.
- Additional **parameters** that are passed to the shell script in our case. They tell the script the percentage that
  has been reached (e.g. 95) and the address of the user who should get the warning.

Apparently we need the script to run. So please create a new file at `/usr/local/bin/quota-warning.sh` and put these
lines into it:

```
#!/bin/sh
PERCENT=$1
USER=$2
cat << EOF | /usr/lib/dovecot/dovecot-lda -d $USER -o "plugin/quota=maildir:User quota:noenforcing"
From: postmaster@webmail.example.org
Subject: Quota warning - $PERCENT% reached

Your mailbox can only store a limited amount of emails.
Currently it is $PERCENT% full. If you reach 100% then
new emails cannot be stored. Thanks for your understanding.
EOF
```

Make this file executable:

```
chmod +x /usr/local/bin/quota-warning.sh
```

Time to restart Dovecot again:

```
systemctl restart dovecot
```

Dovecot’s quota limits can be configured in many ways. If you have special needs then give
[their documentation](https://doc.dovecot.org/configuration_manual/quota/) a look.
