IMAP Backup Tool: What to Actually Look For

There are a lot of IMAP email backup and archiving tools out there, ranging from free open-source utilities to enterprise archiving suites costing thousands per year. Most marketing pages look nearly identical — "secure," "reliable," "easy backups." The differences that actually matter are underneath the marketing copy, in decisions about file format and data ownership that you usually only discover after you've already committed to a tool. Here's what to actually check.

1. What file format does it actually save?

This is the single most important question, and the one most people skip. Ask: if I stopped using this tool tomorrow, could I open my archived emails with something else?

Individual .eml files (RFC 5322) — the best outcome. Each email is its own standard file that any mail client, or even a text editor, can open. Nothing is locked in.
Mbox — a single large file per folder with every message concatenated. Standard and widely supported, but not directly browsable without importing it somewhere.
Proprietary database or container format — the worst outcome. Your archive is only usable through that specific tool. If the company shuts down or you stop paying, you may lose practical access to your own backup.

2. Where does the archive actually live?

Local-first tools save files directly to a folder on your disk that you control. Cloud-based archiving services store your mail on their servers instead, which means your backup's availability depends on that company staying in business and your subscription staying current. Neither approach is inherently wrong, but you should know which one you're getting — a "backup" that disappears if you stop paying isn't really a backup, it's a subscription.

3. Does it handle authentication properly?

For Gmail and Microsoft accounts specifically, the tool should support OAuth2 sign-in rather than asking for your raw password or an app password — both providers have been moving away from password-based IMAP access for security reasons, and some accounts don't support it at all anymore. For providers like Yahoo that don't offer app passwords, the tool needs to support their standard sign-in flow instead; if a tool's only Yahoo option is "generate an app password," it hasn't kept up with how Yahoo's authentication actually works today.

4. Can you selectively archive, or is it all-or-nothing?

Good tools let you pick specific folders, labels, or date ranges. This matters more than it sounds — if you only want to archive and clear out email older than two years, an all-or-nothing export forces you to download (and manually sort through) everything, including mail you didn't need to touch.

5. Does it preserve attachments and formatting faithfully?

Test this before trusting a tool with anything important: archive a handful of emails with attachments and HTML formatting, then open them back up. Attachments should be intact and openable, and HTML emails should render close to how they looked originally, not stripped down to plain text.

6. Is the source code auditable, or are you trusting a black box?

Email is sensitive by nature — financial records, personal correspondence, work communication. Open-source tools let you (or someone you trust) actually verify what the software does with your data, rather than taking a vendor's word for it. This matters more for a backup tool than almost any other category of software, because you're specifically handing it your most private information.

7. What does it cost to keep using your own backup?

Some archiving services charge ongoing fees just to keep your existing backup accessible, effectively renting your own data back to you. A tool that saves to open, standard files on your own disk doesn't have this problem — your archive keeps working with or without the tool once it's saved.

Where MailVault fits

MailVault is one option in this category, and it's worth judging against the checklist above rather than taking that on faith: it's free and open source, it saves individual .eml files in a standard Maildir structure on your own disk (not a proprietary format or someone else's server), it supports OAuth for Gmail and Microsoft and Yahoo's standard sign-in flow, and it lets you selectively archive specific folders. Manual archiving is unlimited and free forever; scheduled automatic backups, cross-account migration, cleanup rules, and point-in-time snapshots are premium features at $4/month or $25/year, with a 14-day free trial on the yearly plan. It currently runs on macOS and Linux, not Windows.

See how MailVault stacks up against this checklist

Individual .eml files, your own disk, open source, OAuth where it matters. Free to try, unlimited manual backups always free.

Download MailVault See pricing

Frequently Asked Questions

What's the difference between .eml and .mbox for email backups?

An .eml file holds one email and can be opened individually by almost any mail client. An .mbox file bundles many emails into a single large file, which is standard but not directly browsable without importing it into a client first.

Is a cloud-based email archiving service the same as a local backup?

No. A cloud archiving service stores your mail on the vendor's servers, so access depends on staying subscribed and the vendor staying in business. A local backup saves files to a disk you control, independent of any ongoing subscription.

Why does OAuth support matter for an IMAP backup tool?

Gmail and Microsoft accounts increasingly require OAuth2 sign-in rather than a raw password, and some accounts don't support password-based IMAP access at all. A tool without proper OAuth support may simply fail to connect to these accounts.

Should an email backup tool be open source?

It's a strong plus, not a strict requirement. Because email backups involve highly sensitive personal and financial information, open-source code lets you or someone you trust verify what the tool actually does with your data instead of relying on trust alone.