Postbox 6 Product Info



Postbox 6 is the first release of our next-generation platform. Even though it looks and acts like previous Postbox releases, everything under the covers has changed. Postbox 6 has an entirely new foundation that will improve performance, message rendering, security, memory utilization, and reliability. Think of this release as "next generation Postbox 1.0."

We're at the very beginning of our journey with this new foundation. With any major release, bugs will pop up that we didn't find during the beta period, and some features that you may be used to in previous versions of Postbox may not be present in Postbox 6.

We have a lot of exciting features on the horizon, so we hope that you are as excited about the possibilities as we are.

Migrating from Postbox 3, 4 or 5.

Please see our Postbox 6 Migration Guide.

IMAP Limiting

Version 6 can now limit the number of messages that Postbox will fetch per folder. This feature is turned "ON" by default and is currently set at 2,000 messages (adjustable to 1,000, 2,000, 5,000, 10,000 or no limit).

You can increase these limits so see more messages in Preferences (macOS) or Options (Windows) > Accounts > [your account] > Local Storage.


Postbox is based on Mozilla code, and as of Firefox Quantum, add-ons have been removed from the Mozilla platform. Since future versions of Postbox will no longer have the ability to run add-ons, we've discontinued support for them in Postbox 6.

Add-ons developed by Postbox, Inc. have been added to Postbox 6, including Signatures, Responses, and our Cloud File Sharing.

3rd party add-ons will no longer operate in Postbox 6. We understand this will be disappointing for many, but we cannot maintain add-ons compatibility over the long-term if they are not supported by underlying development platform.

Will the functionality of your favorite add-on be rolled into Postbox? Very possible, because after all, we want to make Postbox the very best it can be. But please note that we do not make announcements or commitments on future features until they're ready to be tested in a public BETA release. If you have ideas on the features Postbox should support, please tell us about them on our Feature Request page.


Postbox 6 contains a new dictionary management system. To install a Mozilla dictionary, download the dictionary to disk then go to Preferences (macOS) or Options (Windows) > Composition > Typing > Edit Dictionaries. Navigate to the downloaded dictionary file to install it.

Your default dictionary preference can be set in Preferences (macOS) or Options (Windows) > Composition > Typing > Language.

Within the Compose window, you can quickly switch dictionaries for the current compose session by clicking and holding the Spelling toolbar button. Once you send, the dictionary will reset to its default language.

Please note that you cannot install Mozilla Language Packs in Postbox.

Hidden Settings

Most of the hidden preferences that customers have accessed through the Config Editor are now available in the UI. As such, we've removed the Config Editor from Postbox 6. We'll be adding more preferences to the UI over time.

Domain Fencing

By popular demand, we've adjusted the colors of our Domain Fencing feature. Grey addressing bubbles are the default, and red addressing bubbles will provide a warning if you're about to send to or from the wrong domain.

How to Purchase an Upgrade

If you purchased a Postbox 5 license key on or after March 1st, 2016, or a Postbox Lifetime Key on or before July 31st, 2010, then you already have a Lifetime License, and no purchase is necessary. Download and install Postbox 6, and you're good to go!

All other customers must purchase a license to use Postbox 6.




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