The place for Postbox news and product updates.

April 3rd, 2018

​Postbox 6.0

Posted in: News, Updates, Product


We're excited to announce the immediate availability of Postbox 6 for Mac and PC, our next-generation email platform.

While Postbox 6 may feel familiar, under the covers it has an entirely new foundation that will improve performance, message rendering, security, memory utilization, and reliability. More vivid colors, updated themes for macOS and Windows, and over twice as fast as Postbox 5, we know you'll be as excited about this release that we are.

There are a few changes that current Postbox users should know about before installing Postbox 6, so please read on!

Migrating from Older Versions

Postbox 6 installs into new data directories which allows it to run concurrently with older versions of Postbox. If you are using version 3, 4 or 5, Postbox will offer to migrate your accounts, local mail data, user generated content, and most user preferences over from your previous install to version 6.

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 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.

Within the Compose window, you can quickly switch dictionaries by clicking and holding the Spelling toolbar button.


Postbox is built using Mozilla code, and as of Firefox Quantum, add-ons are no longer supported by the Mozilla platform. Consequently, future versions of Postbox will no longer support add-ons, starting with Postbox 6.

Add-ons from Postbox, Inc. such as Cloud File Sharing, Signatures, and Responses have been added to the core Postbox 6 product.

We fully understand that this change will be disappointing for many add-ons users, we genuinely do. But we cannot move Postbox forward while maintaining backward compatibility for an add-ons infrastructure that is no longer supported.

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, so if there's a specific preference that you are interested in, please let us know.

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 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!


A special "Thank You" to all of our amazing BETA testers who assisted in the development of Postbox 6, we couldn't have done it without you!