Dovecot IMAP/POP3 Server

Overview

• What you’ll learn: How to install and configure Dovecot as an IMAP/POP3 server on Ubuntu — including mailbox formats, TLS encryption, user authentication, and integration with Postfix for a complete mail system.
• Prerequisites: Email Protocols (SMTP, IMAP, POP3), Postfix Mail Transfer Agent, basic Linux administration
• Estimated reading time: 18 minutes

Introduction

While Postfix handles the sending and receiving of email via SMTP, users need a way to access their mailboxes from email clients such as Thunderbird, Outlook, or mobile apps. This is where Dovecot comes in. Dovecot is a high-performance, security-focused IMAP and POP3 server that allows users to retrieve and manage their email stored on the server.

Dovecot is the most widely used IMAP/POP3 server on Linux and is the default choice on Ubuntu. It integrates seamlessly with Postfix, providing both mailbox delivery (via the Dovecot LDA or LMTP) and SASL authentication for SMTP clients.

In this lesson, you will learn how to install Dovecot, configure it for IMAP access with TLS encryption, set up mailbox storage using the Maildir format, and integrate it with Postfix to create a functional mail server stack.

Installing Dovecot

Dovecot is available in Ubuntu’s default repositories. You typically install the core package along with the IMAP module. If you also need POP3 support, install the POP3 module as well.

# Install Dovecot with IMAP and POP3 support
$ sudo apt update
$ sudo apt install dovecot-core dovecot-imapd dovecot-pop3d -y

# For LMTP delivery integration with Postfix
$ sudo apt install dovecot-lmtpd -y

# Verify installation and version
$ dovecot --version
2.3.16 (7e2e900c1a)

# Check service status
$ sudo systemctl status dovecot
● dovecot.service - Dovecot IMAP/POP3 email server
     Loaded: loaded (/lib/systemd/system/dovecot.service; enabled)
     Active: active (running)

# Verify Dovecot is listening on IMAP and POP3 ports
$ sudo ss -tlnp | grep dovecot
LISTEN  0  100  0.0.0.0:143   0.0.0.0:*  users:(("dovecot",pid=2345,fd=21))
LISTEN  0  100  0.0.0.0:993   0.0.0.0:*  users:(("dovecot",pid=2345,fd=23))
LISTEN  0  100  0.0.0.0:110   0.0.0.0:*  users:(("dovecot",pid=2345,fd=25))
LISTEN  0  100  0.0.0.0:995   0.0.0.0:*  users:(("dovecot",pid=2345,fd=27))

Dovecot Configuration Structure

Dovecot uses a modular configuration structure. The main configuration file is /etc/dovecot/dovecot.conf, which includes files from /etc/dovecot/conf.d/. Each file in conf.d/ handles a specific aspect of configuration:

• 10-auth.conf: Authentication mechanisms and password databases.
• 10-mail.conf: Mailbox location and format settings.
• 10-master.conf: Service definitions (ports, sockets, processes).
• 10-ssl.conf: TLS/SSL certificate and protocol settings.
• 15-lda.conf: Local Delivery Agent configuration.
• 20-imap.conf / 20-pop3.conf: Protocol-specific settings.

# List all configuration files
$ ls /etc/dovecot/conf.d/
10-auth.conf  10-mail.conf  10-master.conf  10-ssl.conf
15-lda.conf   15-mailboxes.conf  20-imap.conf  20-pop3.conf
90-quota.conf  auth-system.conf.ext

# View the effective configuration
$ doveconf -n

Configuring Mailbox Storage

Dovecot supports multiple mailbox formats. The two most common are mbox (all messages in a single file) and Maildir (each message as a separate file in a directory structure). Maildir is strongly recommended for production use because it is more reliable, does not require file locking, and performs better with IMAP.

# Edit /etc/dovecot/conf.d/10-mail.conf

# Set Maildir as the mailbox format
# Messages stored in ~/Maildir/ for each user
mail_location = maildir:~/Maildir

# Automatically create and subscribe to default folders
namespace inbox {
  inbox = yes

  mailbox Drafts {
    auto = subscribe
    special_use = Drafts
  }
  mailbox Sent {
    auto = subscribe
    special_use = Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = Trash
  }
  mailbox Junk {
    auto = subscribe
    special_use = Junk
  }
}

Ensure this matches the home_mailbox setting in Postfix’s main.cf:

# In /etc/postfix/main.cf — must match Dovecot's mail_location
home_mailbox = Maildir/

TLS Encryption

All IMAP and POP3 connections should be encrypted with TLS. Dovecot supports both STARTTLS (upgrading a plaintext connection) and implicit TLS (encrypted from the start on ports 993/995).

# Edit /etc/dovecot/conf.d/10-ssl.conf

# Require SSL/TLS
ssl = required

# Certificate and key paths (using Let's Encrypt)
ssl_cert = </etc/letsencrypt/live/mail.example.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.example.com/privkey.pem

# Minimum TLS version
ssl_min_protocol = TLSv1.2

# Prefer server cipher order
ssl_prefer_server_ciphers = yes

# Strong cipher suite
ssl_cipher_list = ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384

Postfix Integration via LMTP and SASL

For a complete mail stack, Dovecot integrates with Postfix in two ways: delivering mail via LMTP (Local Mail Transfer Protocol) and providing SASL authentication so Postfix can verify SMTP client credentials.

# --- Dovecot LMTP Service ---
# Edit /etc/dovecot/conf.d/10-master.conf

service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
  }
}

# --- Dovecot SASL for Postfix ---
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

# --- Postfix side: /etc/postfix/main.cf ---
# Use Dovecot LMTP for local delivery
mailbox_transport = lmtp:unix:private/dovecot-lmtp

# Use Dovecot for SASL authentication
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_auth_enable = yes

After making changes, restart both services:

# Restart Dovecot and Postfix
$ sudo systemctl restart dovecot
$ sudo systemctl restart postfix

# Test IMAP login
$ openssl s_client -connect mail.example.com:993
a1 LOGIN user@example.com password
a1 OK [CAPABILITY IMAP4rev1 ...] Logged in

# Test with doveadm
$ sudo doveadm mailbox list -u user@example.com
INBOX
Sent
Drafts
Trash
Junk

Key Takeaways

• Dovecot is the standard IMAP/POP3 server on Linux, providing mailbox access for email clients; it uses a modular configuration structure under /etc/dovecot/conf.d/.
• The Maildir format (one file per message) is recommended over mbox for production use; set mail_location = maildir:~/Maildir and match Postfix’s home_mailbox.
• TLS should be set to required with a minimum of TLSv1.2; use Let’s Encrypt certificates for both Dovecot and Postfix.
• Dovecot integrates with Postfix via LMTP for mail delivery and SASL for SMTP authentication, using Unix sockets in Postfix’s chroot directory.
• Use doveconf -n to verify effective configuration and doveadm for mailbox management, user lookup, and troubleshooting.

What’s Next

In the next lesson, you will learn about email security mechanisms — SPF, DKIM, and DMARC — that protect your domain from spoofing and improve mail deliverability.

# 安裝具有 IMAP 和 POP3 支援的 Dovecot
$ sudo apt update
$ sudo apt install dovecot-core dovecot-imapd dovecot-pop3d -y

# 用於與 Postfix 的 LMTP 投遞整合
$ sudo apt install dovecot-lmtpd -y

# 驗證安裝和版本
$ dovecot --version
2.3.16 (7e2e900c1a)

# 檢查服務狀態
$ sudo systemctl status dovecot

# 編輯 /etc/dovecot/conf.d/10-mail.conf

# 設定 Maildir 作為信箱格式
mail_location = maildir:~/Maildir

# 自動建立和訂閱預設資料夾
namespace inbox {
  inbox = yes
  mailbox Drafts {
    auto = subscribe
    special_use = Drafts
  }
  mailbox Sent {
    auto = subscribe
    special_use = Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = Trash
  }
}

# 編輯 /etc/dovecot/conf.d/10-ssl.conf
ssl = required
ssl_cert = </etc/letsencrypt/live/mail.example.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.example.com/privkey.pem
ssl_min_protocol = TLSv1.2
ssl_prefer_server_ciphers = yes

# Dovecot LMTP 服務 — 編輯 /etc/dovecot/conf.d/10-master.conf
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
  }
}

# Dovecot SASL for Postfix
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

# 重新啟動兩個服務
$ sudo systemctl restart dovecot
$ sudo systemctl restart postfix

# IMAP と POP3 サポート付きで Dovecot をインストール
$ sudo apt update
$ sudo apt install dovecot-core dovecot-imapd dovecot-pop3d -y

# Postfix との LMTP 配信統合用
$ sudo apt install dovecot-lmtpd -y

# インストールとバージョンの確認
$ dovecot --version
2.3.16 (7e2e900c1a)

# サービス状態の確認
$ sudo systemctl status dovecot

# /etc/dovecot/conf.d/10-mail.conf を編集

# Maildir をメールボックス形式として設定
mail_location = maildir:~/Maildir

# デフォルトフォルダを自動作成・購読
namespace inbox {
  inbox = yes
  mailbox Drafts {
    auto = subscribe
    special_use = Drafts
  }
  mailbox Sent {
    auto = subscribe
    special_use = Sent
  }
  mailbox Trash {
    auto = subscribe
    special_use = Trash
  }
}

# /etc/dovecot/conf.d/10-ssl.conf を編集
ssl = required
ssl_cert = </etc/letsencrypt/live/mail.example.com/fullchain.pem
ssl_key = </etc/letsencrypt/live/mail.example.com/privkey.pem
ssl_min_protocol = TLSv1.2
ssl_prefer_server_ciphers = yes

# Dovecot LMTP サービス — /etc/dovecot/conf.d/10-master.conf を編集
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    mode = 0600
    user = postfix
    group = postfix
  }
}

# Postfix 用 Dovecot SASL
service auth {
  unix_listener /var/spool/postfix/private/auth {
    mode = 0660
    user = postfix
    group = postfix
  }
}

# 両方のサービスを再起動
$ sudo systemctl restart dovecot
$ sudo systemctl restart postfix