The 3-2-1 backup rule, applied to your email

The 3-2-1 rule says to keep three copies of your data, on two different kinds of storage, with one copy somewhere other than your home or office. For email, copy one is the mailbox you already have at Gmail, Outlook or your web host. The other two are yours to set up: say, an external drive at home and a cloud storage bucket that isn’t run by your email provider.

That can sound like overkill for “just email”. Then you remember what’s actually in there: a decade of receipts, contracts, tax documents, client threads and password reset links. Here’s how to apply the rule without turning it into a weekend project.

What each number means

Three copies. The original plus two backups. With a single backup, the day that drive dies you’re back to square one. With two, one can fail and you still have the other.

Two kinds of storage. The copies shouldn’t share a single point of failure. Two folders on the same hard drive aren’t two copies; if the drive goes, both go. An internal drive and an external drive count as different media. So do a NAS and a cloud bucket.

One off-site. A fire, a flood or a burglary takes everything in the same room, including the laptop and the USB drive sitting next to it. A copy in another building, or in cloud object storage, covers that.

Your provider’s mailbox counts, but only once

This is where most people trip up. Your mail lives “in the cloud”, so the off-site part feels handled. Google’s and Microsoft’s servers are certainly far from your desk. But that mailbox is the original, not a copy. Delete a message and it’s deleted there. Someone breaks into your account and empties folders, they’re emptied there. The account gets suspended or closed, the whole thing is gone.

We go through this in more detail in Cloud email is not a backup. For 3-2-1 purposes, the mailbox is copy number one, and you still need two more.

The same goes for desktop mail apps that sync over IMAP, like Apple Mail, Outlook or Thunderbird. They keep a local copy, but it mirrors the server: whatever is deleted upstream disappears locally on the next sync. Great for reading offline, not for getting lost mail back.

Setting up 3-2-1 for your email

A sensible setup, from least to most effort:

1. Copy 1: the mailbox. You already have it.
2. Copy 2: a drive you control. An external drive, or a second internal drive that isn’t your system disk. Make it automatic. A backup that depends on you remembering ends up happening twice a year.
3. Copy 3: off-site. A NAS at another location (the office, a relative’s place) or S3-compatible storage such as Backblaze B2, Wasabi, Cloudflare R2 or Hetzner. Object storage is inexpensive at the sizes most mailboxes reach, and it doesn’t need a machine running somewhere else.

If that feels like a lot, start with copy 2. Going from zero backups to one is the biggest jump in protection you’ll get. Add the off-site copy when you can.

A backup that deletes when the original deletes doesn’t count

For copies 2 and 3 to be worth anything, they need something the 3-2-1 rule doesn’t spell out: they must not mirror deletions. If your backup tool removes messages from the backup when they vanish from the server, you have three copies of the same problem.

Think about format too. If the tool that made the backup isn’t around in five years, can you still open it? With an open format like mbox, yes: Thunderbird, Apple Mail and plenty of other programs read it. What is an mbox file? explains why that matters.

Common mistakes

• Dropping the backup into a synced folder. Dropbox, OneDrive and iCloud Drive replicate whatever happens on your computer. If ransomware encrypts your files, the encrypted versions get uploaded too. Some services keep earlier versions for a while; check what yours does.
• Leaving the external drive plugged in forever. Convenient, but whatever hits your computer can hit the drive. For the off-site copy, prefer something that isn’t mounted as just another folder.
• Treating an old export as your backup. A Google Takeout from 2024 is a fine keepsake. It does nothing for the mail that arrived since. We compare the two approaches in Google Takeout vs. automatic backup.
• Never testing a restore. A backup you’ve never opened is an assumption. At least once, search it for an old message and open it.

What about your phone?

Your phone doesn’t count. Mail apps keep a small cache of recent messages and sync with the server. Real backups are made from a computer.

How much space does it take?

Less than you’d think. Your provider shows how much storage your mailbox uses, attachments included, and the backup will be roughly that size. For most personal mailboxes it fits easily on an ordinary external drive. Object storage bills by usage, so check the pricing of whichever service you pick.

Where Arca fits in

Arca Mail Backup was built with this rule in mind. It connects over IMAP to Gmail, Google Workspace, Outlook.com, Microsoft 365 or any web host, and backs up your mail in the background as standard mbox files, one per folder and month, that you can open without the app. Mail deleted on the server stays in your backup, marked as deleted.

The free plan backs up one account to an internal or external drive, once a day. That’s your copy 2. Pro adds NAS and S3-compatible destinations (Wasabi, Backblaze B2, Cloudflare R2, Hetzner) and can write to several destinations at once, which is your copy 3. Pro yearly starts at $8.99 for one account, and there’s a $44.99 lifetime option for one account; see pricing. Closed months never change, so they play well with bucket versioning and Object Lock.

Arca for Windows is out now in the Microsoft Store (Windows 10 version 1809 or later, or Windows 11). The Mac version (macOS 14 or later) is in the Mac App Store. Arca Pro is on sale in both stores.