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What is the most innovative and convenient way to set up delivery and manage virtual users on a Postfix mail server capable of handling 100,000 to 1,000,000 users?

  • Mbox scares me because reading one file for an entire INBOX is an intensive process, right?
  • Maildir is pretty cool but what if I have to do some maintenance? Then I'll have to worry about writing scripts to traverse multiple files.
  • Are there any SQL solutions for storing this much mail or does that inflate the database too much thus making it cumbersome to backup?
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Your questions and your possible answers mismatch.

The handling of virtual users in Postfix is best done via either a database (DBMS) or LDAP. MySQL with only 1,000,000 users is just fine.

But your answers are a problem of your mailbox accessing software (IMAP or POP3 server). This is in no way related to Postfix nor a problem of your user base. It is dependent of the mailbox size and mailbox access. If you only provide POP3, then mbox is fine because the users normally fetch the complete mailbox and then delete all mails. Maildir is my preferred mailbox storage. You can do incremental backups and have access to each mail. You can even spread the mailboxes on different networking file systems. Databases for mail storage is a real challenge. Can you handle database tables with 1,000,000 times 1GB in size? Concerning backups, restore, fragmentation, ...

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