Dovecot -- IMAP, POP3, and Authentication¶

Dovecot is where emails live once they arrive. It stores them, serves them to mail clients via IMAP and POP3, manages quotas, runs Sieve filter scripts, and -- importantly -- handles authentication for the entire mail system (including Postfix).

What It Does¶

• IMAP on ports 143 (STARTTLS) and 993 (implicit TLS)
• POP3 on ports 110 (STARTTLS) and 995 (implicit TLS)
• LMTP on port 24 -- receives mail from Postfix for local delivery
• Auth service on port 24100 -- shared SASL authentication for Postfix
• ManageSieve on port 4190 -- remote Sieve script management
• Quota enforcement with warning notifications at 75%, 80%, and 95%
• Smart folders -- auto-created Notifications, Social, Promotions, Updates folders

Architecture¶

flowchart TB
    subgraph Dovecot Container
        IMAP["imap-login\n:143 / :993"]
        POP3["pop3-login\n:110 / :995"]
        LMTP["LMTP\n:24"]
        AUTH["Auth Service\n:24100"]
        SIEVE["ManageSieve\n:4190"]

        IMAP --> IMAP_PROC[imap process]
        POP3 --> POP3_PROC[pop3 process]

        LMTP --> SIEVE_PLUGIN[Sieve Plugin]
        SIEVE_PLUGIN --> MAILDIR[Maildir Storage]

        IMAP_PROC --> MAILDIR
        POP3_PROC --> MAILDIR

        AUTH --> AUTH_WORKER[auth-worker]
        AUTH_WORKER --> SQL_DRIVER[SQL Driver]

        QUOTA[Quota Plugin] -.-> MAILDIR
    end

    Postfix -->|"LMTP :24"| LMTP
    Postfix -->|"SASL :24100"| AUTH
    SQL_DRIVER --> MySQL[(MySQL)]
    AUTH_WORKER -->|"policy check"| POLICY["Auth Policy\n:8090"]

Configuration¶

Everything is in a single file: mailer/dovecot/config/dovecot.conf. Dovecot does support split configs with conf.d/ includes, but we keep things in one place for clarity.

Authentication¶

auth_mechanisms = plain login
auth_username_format = %Lu          # lowercase the username
auth_cache_size = 10M               # cache auth results for performance
auth_cache_ttl = 1 hour
auth_cache_negative_ttl = 1 min     # cache "wrong password" for 1 min
auth_failure_delay = 2 secs         # slow down brute-force attempts

Credentials come from MySQL via the SQL driver:

passdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}
userdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}

The dovecot-sql.conf.ext file contains the actual SQL queries for looking up passwords and user info from the email_accounts table.

Auth Policy (Brute-Force Protection)¶

Dovecot integrates with a Redis-backed auth policy server for brute-force detection:

auth_policy_server_url = http://127.0.0.1:8090/
auth_policy_check_before_auth = yes
auth_policy_check_after_auth = yes
auth_policy_report_after_auth = yes

The policy server (dovecot-auth-policy.py) tracks failed login attempts per IP and username, progressively increasing delays and eventually blocking repeat offenders.

Virtual Users and Mail Storage¶

All mailboxes use Maildir format, stored on a shared volume:

mail_location = maildir:/var/mail/vhosts/%d/%n
# %d = domain (example.com)
# %n = user (john)
# Result: /var/mail/vhosts/example.com/john/

The vmail user (UID/GID 5000) owns all mail files. No system accounts are needed for virtual users.

Namespace and Special Folders¶

Dovecot auto-creates standard IMAP folders on first login:

The last four are smart folders -- Rspamd classifies incoming email with an X-Email-Category header, and a global Sieve script routes them accordingly.

Quota Management¶

Quotas use the Maildir backend (counts actual disk usage):

quota = maildir:User quota
quota_rule  = *:storage=5GB          # Default: 5 GB per mailbox
quota_rule2 = Trash:storage=+500MB   # Trash gets extra 500 MB
quota_rule3 = Junk:storage=+100MB    # Junk gets extra 100 MB
quota_grace = 10%%                   # 10% grace period

Per-user quotas from the database override these defaults via the quota_rule field in the SQL user query.

Warning thresholds trigger an external script:

quota_warning  = storage=95%% quota-warning 95 %u
quota_warning2 = storage=80%% quota-warning 80 %u
quota_warning3 = storage=75%% quota-warning 75 %u

The dovecot-quota-warning.sh script sends a notification email to the user and can trigger webhook events for admin alerting.

Sieve Filtering¶

Sieve lets users (and the system) define mail filtering rules:

sieve = file:~/sieve;active=~/.dovecot.sieve
sieve_global_dir = /etc/dovecot/sieve/global/
sieve_global_path = /etc/dovecot/sieve/default.sieve
sieve_extensions = +notify +imapflags +vacation-seconds +editheader

The global Sieve script runs for every email and handles smart folder routing. Per-user scripts can add vacation replies, forwarding rules, and custom filtering.

Users manage their Sieve scripts remotely via ManageSieve on port 4190.

TLS Configuration¶

ssl = required                    # No plaintext connections
ssl_cert = </etc/ssl/certs/server.crt
ssl_key  = </etc/ssl/private/server.key
ssl_dh   = </etc/dovecot/dh.pem  # DH params for forward secrecy
ssl_min_protocol = TLSv1.2
ssl_prefer_server_ciphers = yes

SNI support for multiple domains:

!include_try /etc/dovecot/conf.d/sni.conf

The cert manager writes per-domain local_name {} blocks to this file and sends SIGHUP to Dovecot to reload without dropping connections.

Service Definitions and Process Limits¶

All login processes run as dovenull in a chroot jail for security. The auth service runs as dovecot with a dedicated pool of workers.

Protocol-Specific Settings¶

IMAP¶

mail_max_userip_connections = 20   # Max connections per user per IP
imap_idle_notify_interval = 2 mins # IDLE push notification interval

POP3¶

pop3_uidl_format = %08Xu%08Xv     # Unique message IDs
pop3_fast_size_lookups = yes       # Performance optimization
pop3_lock_session = yes            # Prevent concurrent POP3 sessions

Health Checks¶

The monitoring service checks Dovecot by:

1. Connecting to IMAP port 993 and verifying the banner
2. Checking the auth service is responding on port 24100
3. Verifying LMTP delivery works on port 24
4. Monitoring doveadm stats for unusual patterns

Docker Configuration¶

dovecot:
  build: ./mailer/dovecot
  container_name: dovecot
  ports:
    - "143:143"
    - "993:993"
    - "110:110"
    - "995:995"
  volumes:
    - mail_data:/var/mail/vhosts
    - ssl_certs:/etc/ssl
    - sieve_scripts:/etc/dovecot/sieve

Environment Variables¶

Gotchas¶