But since my customers needed to be able to read and reply to emails while the primary site was down, I couldn’t just let the backup system keep the incoming email in the queue and deliver them once the primary sit was up. I decided to deliver the emails locally but also keep a copy of all emails that passed through the system.

These archived emails were to be re-delivered once the primary site was up again, and here’s how I did it, step by step.

• Set up a VPS for the backup MX host.
• Set up the system using the same guide I used for the primary site, which was the great ISPmail tutorial by Christoph Haas.
• Added the option always_bcc to the Postfix config, pointing it to a new local mailbox.
• Once the primary server was up again, I set up getmail like this:

  • [retriever]
    type = SimplePOP3Retriever
    server = hostname_of_backup_mx
    username = username_of_backup_account
    password = secret

    [destination]
    type = MDA_external
    arguments = ("-i", "-bm", "-t")

    [options]
    verbose = 2

Running getmailrc quickly restored all the emails on the primary host. Remember not to run it multiple times since the default is to re-download already processed messages.

The downsides of this setup:

• Deleted emails (like spam) are seen again by the users. POP users will receive duplicates of already retrieved messages. IMAP users will however have a mailbox which includes both old messages and the ones delivered to the backup server.
• The spam filter wasn’t used in delivery process. I realize this could be achieved with fetchmail (which includes the SMTP delivery option that getmail lacks) but I decided to use getmail anyway.

I realize there might be prettier solutions to the original problem (BOTH delivering emails locally and keeping them in a queue for a primary MX), but I couldn’t find out how and this proved to be a pretty solid solution. Feel free to suggest better ways to solve the problem!