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Version 1.6.0 Available Now

Mundofy MailMigrator

Open-source mailbox migration tool for Windows and macOS, built in C# on .NET 8 and licensed under the MIT License.

Built out of our daily hosting operations and released free for everyone. Inspired by Armin Diehl's classic imapcopy, it transfers mailboxes from IMAP or POP3 sources to IMAP destination servers with native TLS 1.3, duplicate skipping, resume support, desktop interfaces for Windows and macOS, and command-line automation.

Download release on GitHub
View documentation on GitHub Wiki
View repository on GitHub
Mundofy MailMigrator open source mailbox migration tool interface

Built to modernize mailbox migrations

In managed hosting operations, moving customer mailboxes to new servers is a routine task. Email migrations present operational challenges, particularly when command-line experience is limited and avoiding lost messages or downtime is essential.

For years, Armin Diehl's classic open-source imapcopy utility (2001–2009) was the gold standard for this work. It was simple, reliable, and sysadmins everywhere swore by it. Over time, however, mail servers dropped older protocols and strictly enforced TLS 1.3, causing connection handshakes to fail without proxy daemons like stunnel. Classic imapcopy also lacked POP3 support, multi-threading, and modern error recovery.

Rather than keeping our internal migration scripts private, we built Mundofy MailMigrator to combine the straightforward workflow of classic utilities with native TLS 1.3 encryption, POP3 source support, multi-account concurrency, and desktop interfaces alongside the command-line interface. It isn't a commercial product; it is an operational tool we use daily, released as free open-source software under the MIT License for everybody to use.

Migration capabilities and protocols

Protocol support

Connects to IMAP (port 993 or 143) or POP3 (port 995 or 110) as a source and transfers mail to IMAP (port 993 or 143) destinations with native TLS 1.3 and SSL encryption. The protocol engine is powered by Jeffrey Stedfast's MailKit library.

Duplicate skipping and resume

Checks RFC 822 Message-ID headers against destination folders before moving each message. Skips existing messages and allows interrupted transfers to resume without creating duplicate emails.

Server auto-discovery

Detects IMAP server hostnames, port 993, and SSL encryption from an email address using a five-tier discovery cascade (Major Providers, RFC 6186 DNS SRV, DNS MX Fingerprinting, Mozilla Thunderbird ISPDB, and cPanel/Plesk Autoconfig).

Mailbox quota checking

Queries source and destination mailbox storage limits and usage before migration using RFC 2087 IMAP QUOTA and POP3 metrics, alerting you if destination capacity is insufficient.

Multi-account concurrency

Migrates multiple accounts in parallel with configurable concurrency ranging from 1 to 16 simultaneous workers.

Backward compatibility

100% compatible with classic ImapCopy.cfg configuration files. Also supports importing and exporting standard CSV and TSV account lists, pasting rows from the clipboard, and per-account pause and resume.

System sleep prevention

Prevents the computer from entering sleep mode during active data transfers while allowing connected monitors to turn off.

Zero-telemetry & privacy by design

Direct point-to-point TLS 1.3 sockets between your computer and mail servers. Pure in-memory streaming with zero cloud relays, zero disk caching, and zero tracking, making it inherently compliant with GDPR, CCPA, and HIPAA.

Interactive desktop GUI and headless CLI

Desktop application

Double-clicking MundofyMailMigrator.exe on Windows, or launching the application bundle on macOS, opens an interactive desktop interface. Users can enter credentials directly on screen for a single mailbox or configure batch migrations in a table. The interface displays folder progress, message counters, transfer speed in MB/s, and progress bars.

Headless command-line interface

For automated migrations and scheduled scripts, the utility runs headlessly from the terminal using standard flags.

Display help: MundofyMailMigrator.exe -h

Run with configuration file: MundofyMailMigrator.exe -c ImapCopy.cfg

Set concurrent workers: MundofyMailMigrator.exe -c ImapCopy.cfg --concurrency 6

Test account logins without transferring: MundofyMailMigrator.exe -c ImapCopy.cfg --test

Releases and source code

Windows standalone executable

Published on GitHub Releases as a single self-contained 64-bit Windows executable (MundofyMailMigrator.exe). Requires no installation and no external runtime dependencies. Note: Because this is a free open-source release without a commercial EV certificate, Windows SmartScreen may show an alert on browser download. Clicking More info and then Run anyway opens the utility.

macOS builds

Universal macOS disk images (.dmg) and application bundles (.app) supporting Apple Silicon and Intel processors are also provided on GitHub Releases.

Building from source

The full source code is available in the GitHub repository under the MIT License. Building the solution requires the .NET 8.0 SDK.

Build solution: dotnet build

Run tests: dotnet test

Publish portable executable: dotnet publish src\Mundofy.MailMigrator.App\Mundofy.MailMigrator.App.csproj -c Release -r win-x64 --self-contained -p:PublishSingleFile=true -o publish\win-x64

View releases on GitHub
Browse documentation on GitHub Wiki

Need us to handle your migration?

If your team prefers not to carry out the migration internally, Mundofy can handle it for you.

We do basic IMAP migrations, Microsoft 365 migrations, and full Nextcloud solutions for you.

Get in touch

Planned software capabilities

These features are in development or planning for upcoming releases of the open-source software tool. Organizations requiring Microsoft 365 or Nextcloud migrations today can engage our assisted migration services above.

Planned: Google Workspace and Microsoft 365 to Nextcloud migration pipeline in the software tool

A direct migration pipeline from Google Workspace and Microsoft 365 into private Nextcloud instances, including direct mailbox transfers into Nextcloud Mail backends, user provisioning via the Nextcloud OCS API, and roadmap expansion into Nextcloud Contacts (CardDAV), Calendar (CalDAV), and Files (WebDAV).

Planned: Native Linux and Docker CLI

A native lightweight Linux binary (linux-x64 and linux-arm64) and an official Docker container for automated server pipelines, continuing the classic imapcopy legacy in modern container environments.

Planned: Smart folder normalization

Automatic mapping between provider folder naming conventions, such as mapping Sent Items to Sent and Deleted Items to Trash.

Planned: Exportable migration audit reports

Generation of CSV and printable HTML audit reports showing mailbox status, bytes transferred, skipped messages, and elapsed duration.

Planned: One-click retry for failed accounts

Re-queue interrupted accounts or failed items without re-evaluating completed mailboxes.

Planned: Configurable bandwidth rate limiting

Adjustable rate limits to prevent server-side connection throttling.

Project credits and open-source license

Heritage & Inspiration

Inspired by Armin Diehl's original imapcopy utility (2001–2009). For years, classic imapcopy was the trusted standard that sysadmins everywhere swore by, and served as the direct inspiration for this modern open-source tool.

Engine

IMAP and POP3 engine powered by Jeffrey Stedfast's open-source MailKit library.

License

Published under the MIT License for free and open use by everyone.

Developed by

Developed by Danny Schuller / Mundofy as an internal migration tool and released free for the open-source community.

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