In parts 1 and 2, we explained the roles of Dovecot and Postfix and how to transition to a virtual user-based system. In this part 3, we will discuss mail management methods and additional data management methods (mail metadata, address book, etc.) on virtual user-based mail servers.

1. Mail Storage Methods

In a virtual user setup, the mail server stores emails in two formats within the file system: Maildir and mbox.

Maildir

In the Maildir format, each email is stored as an individual file. Even with virtual user accounts, emails are stored in a designated directory within the file system, for example, under /var/mail/vmail/domain/user/. Folders such as INBOX, Sent, and Drafts are created and managed automatically.

mbox

In the mbox format, all emails are stored in a single file. In this method, all incoming emails from users are recorded sequentially, managed through the file.

Postfix and Dovecot store mail in Maildir or mbox format after sending and receiving.

2. Database Management of Mail-Related Information

The mail server can store and manage additional data such as mail metadata (subject, sender, recipient, etc.) and user address books in a database. This allows for more efficient management and retrieval of user data.

1) Managing Email Metadata

While the mail itself is stored in the file system, email metadata can be managed in the database. For example, information such as the mail's subject, sender, recipient, and sending time can be stored in a separate table to simplify searching and filtering.

CREATE TABLE email_headers (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES virtual_users(id),
  subject VARCHAR(255),
  sender VARCHAR(255),
  recipient VARCHAR(255),
  date TIMESTAMP,
  is_read BOOLEAN DEFAULT FALSE
);

This table stores basic information about the mail received by users, making mailbox management easier.

2) Address Book Management

The user's address book (Contacts) can be managed in the database, allowing for efficient storage of frequently used email addresses. You can create an address book table like this:

CREATE TABLE contacts (
  id SERIAL PRIMARY KEY,
  user_id INTEGER REFERENCES virtual_users(id),
  contact_name VARCHAR(255),
  contact_email VARCHAR(255)
);

This table allows users to store and manage frequently used email addresses or group information.

3. Mail Search and Indexing

Storing the mail body itself in the database can be inefficient, so the mail body is stored in the file system, while important metadata is stored in the database to optimize search performance.

For this purpose, you can store search keywords such as the mail's subject or sender in the database and manage indexes to quickly locate the mail upon search requests.

4. Dovecot's Sieve Scripts

Dovecot supports mail filtering scripts called Sieve. With Sieve scripts, users can set rules to automatically sort or filter emails upon receipt. For example, you can automatically move mail from a specific sender to an "Important" folder or set up an auto-response mail.

Example: Sieve Script

if address :is "from" "important@example.com" {
  fileinto "INBOX.Important";
}

The script above moves mail to the "Important" folder if the sender is important@example.com. By using Sieve, users can automatically manage and categorize their mailboxes.

5. Mail Management via IMAP

On mail servers operated with virtual user accounts, emails can be stored and managed on the server via IMAP. The advantage of IMAP is that users can synchronize and manage the same mailbox across multiple devices. Connecting to the server via a mail client (e.g., Outlook, Thunderbird) allows real-time synchronization, so when a user reads or deletes a mail, it is reflected on all devices.

Conclusion

On a virtual user-based mail server, mail data can be efficiently managed by combining the file system and database. The mail body is primarily stored in the file system, while metadata and address book data are managed in the database. Additionally, IMAP allows for mail synchronization, and Sieve scripts enable automatic filtering of emails.

This system allows for efficient management of large numbers of users and enhances the performance and scalability of the mail server.