Learn Mailserver - Quota & Mailbox Management

#Introduction

Unlimited email grows without end — and a full disk is the number one disaster on a mail server: the queue jams, logs stop, even the whole system can fail. This episode builds a healthy fence: quotas.

We'll install the quota plugin in Dovecot, set per-user and per-domain limits, reject email when a mailbox is full via the quota-status service, then learn to maintain mailboxes with doveadm and folder cleanup policies.

#The Quota Concept

Quotas limit two things: message count and total size. Limits can be applied per user, per domain, or both. An important design decision: when is the quota checked?

• At SMTP acceptance — the server rejects email before receiving it, saving bandwidth. This is what we build with quota-status.
• At storage — email is accepted then rejected when writing to Maildir. Simpler, but the message has already traversed the network.

The best combination: a quick check at SMTP for most cases, and let Dovecot reject at storage as a safety net.

#Configuring Quotas in Dovecot

Enable the quota plugin in conf.d/10-mail.conf and conf.d/90-quota.conf. With doveadm config set, changes are recorded directly:

doveadm config set mail_plugins 'quota'
doveadm config set quota_driver count
doveadm config set quota_rule 'user=*:storage=2GB:messages=100000'
doveadm config set quota_rule2 'user=*:storage=4GB'
doveadm config set quota_exceeded_message 'Quota exceeded, mailbox is full'

What the rules above mean:

• quota_rule — the main limit: 2 GB per user.
• quota_rule2 — a second rule: an additional 4 GB window (stacking to a 6 GB total) — an example of layered rules.
• quota_exceeded_message — the message returned when the mailbox is full.

For per-domain limits, replace the user=* pattern with a prefix like *@example.com. Reload and check:

sudo dovecot reload
doveconf quota_driver quota_rule

#Reject When Full with quota-status

The quota-status plugin provides a small protocol Postfix can query: "is this user full?" If so, Postfix rejects at the gate before the message enters the queue. Enable the service in conf.d/90-quota.conf:

service quota-status {
  executable = quota-status -p postfix
  unix_listener /var/spool/postfix/private/quota-status {
    mode = 0660
    user = postfix
    group = postfix
  }
  client_limit = 1
}

Then point Postfix at it via the policy service in main.cf:

sudo postconf -e 'smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, check_policy_service unix:private/quota-status'
sudo postfix reload

The new flow: Postfix accepts the email → asks quota-status → if the user is full, it immediately returns 450 4.2.2 Mailbox full without pulling the message. Efficient and queue-protecting.

#Checking Usage with doveadm

doveadm quota is the main window into usage. Use these three variants:

doveadm quota get -u admin@example.com
doveadm quota get -A

The first line shows one user's usage; -A shows all users. For a quick audit of who's nearly full:

doveadm quota get -A | sort -k4 -n

Sorting the doveadm quota get -A output by usage makes it easy to find the users consuming the most space.

#Mailbox Maintenance with doveadm

doveadm mailbox handles per-folder maintenance tasks:

doveadm mailbox status -u admin@example.com messages quota 'INBOX' '*'
doveadm mailbox create -u admin@example.com Archive
doveadm mailbox delete -u admin@example.com 'Trash'

Here's what they mean: the first line shows folder statistics, the second creates a new folder, the third deletes a folder. To count an entire mailbox's size, doveadm mailbox status -u user quota '*' shows total usage across all folders.

#Folder Cleanup Policies

Quotas are most effective when paired with a cleanup policy:

• Trash / Junk — respect their lifetime: Trash and Spam email older than 30 days deserves automatic deletion.
• Sent & Archive — encourage users to store in archives rather than piling up INBOX.
• Sieve cleanup — episode 20 will use the Sieve plugin to move and delete automatically.

Cleanup can run as a cron job calling doveadm:

doveadm expunge -A 'Trash' savedbefore 30d
doveadm expunge -A 'Junk' savedbefore 14d

Run the two lines above as a daily cron — doveadm expunge -A 'Trash' savedbefore 30d deletes Trash messages older than 30 days for all users. A controlled disk is a happy mail server.

#Conclusion

Episode 11 is done. Key takeaways:

• Quotas limit message count and size per user and per domain.
• quota_rule and quota_rule2 build layered limits in Dovecot.
• quota-status lets Postfix reject full mailboxes at the gate without pulling messages.
• doveadm quota get and doveadm mailbox are the primary audit and maintenance tools.
• Regular Trash/Junk folder cleanup keeps the disk healthy.

Space is fenced in. In episode 12 we secure the data: Backup & Migration — backing up Maildir and the database, recovering after damage, and migrating between servers with doveadm sync and imapsync. See you in episode 12!