[2021-07-28]

Quotas

Now this is a feature that caused me quite some headache. But in the end it was pretty simple. Quotas are size limits for users. You can make sure that users do not waste infinite amounts of disk space but are forced to clean up old emails every now and then.

The magic happens in two places. Postfix needs to reject new emails if the user’s mailbox is over quota. And 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 = maildir:User quota

  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 like that but the maildir is the best choice in this context. The string “User quota” is just some random string that may be queried from a mail user agent.

The lines starting with “quota_status_…” set return values for the service that 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 a new 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 responding to requests from other processes. You surely recognize that we put it into the jail that Postfix runs in (/var/spool/postfix). So 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 happily receive new emails and only later get rejected when talking to Dovecot via LMTP. 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 need 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 just a few KB…

# mysql mailserver
mysql> update virtual_users set quota=5000 where email='john@example.org';

…and send him a few test emails. 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.
  • 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.

You still have the 90-quota.conf open in your editor? Good. Add this section to the file (derived from the Dovecot documentation):

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 a look.

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