Postbox BETA 10 Release



Hello, and welcome to the Postbox BETA program!


  • You must have a Postbox 5 license and be running one of these operating systems:
    • macOS 10.11 or higher
    • Windows 7, 8 or 10
  • You must be HIGHLY TECHNICAL, with an ability to troubleshoot issues on your own with no supervision
  • Willing to use a product that is in very early development
  • Using test profiles, test accounts, and test data only
  • Can file detailed bug reports, complete with steps to reproduce and other technical details

If this sounds like more than you bargained for, no problem, simply wait for the production version.

Changes in BETA 10

  • Both macOS and Windows builds are now signed
  • A dramatic improvement in attached image load time

Important Changes in BETA 9! -- PLEASE READ!

Postbox BETA 9 contains the following very important changes:

IMAP Limiting

BETA 9 provides the ability to 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). Postbox will fetch messages based on the order that they were added to the folder, NOT by date.

You can easily increase the limits if you find that you need more messages. The IMAP limiting preferences are per account, and can be adjusted in Preferences (macOS) or Options (Windows) > Accounts > [your account] > Local Storage. We're asking testers to use the 2,000 default so that we receive adequate testing on this feature.

Contacts Optimization on macOS

We've rewritten the code that imports macOS Contacts on startup to improve startup times and reduce memory consumption. 


We've changed the default storage format for new accounts from mbox to Maildir. With mbox, all messages within a folder are stored as one file on disk. Maildir stores each message as a file on disk. This will improve overall reliability, backup capabilities, and handling when mail is quarantined by anti-virus apps. If you migrate from an older version of Postbox, or if you set up a new account, Maildir will be the default storage format. This can be changed in Preferences > Advanced > General.


We have new dictionary management system. To install a Mozilla dictionary, first, download the dictionary to disk. Then go to Preferences (macOS) or Options (Windows) > Composition > Typing > Edit Dictionaries. Then navigate to the dictionary file that you have downloaded.

Other Changes

  • Added more preferences to the Profile Migrator
  • Added several hidden preferences to the UI
  • Support for Mozilla dictionaries
  • Support for macOS Quick Look previews
  • Fix to allow signatures to be manually added
  • Fix for an issue that could occur during indexing
  • Fix to use received date if the date cannot be found in the header
  • Fix to safe browsing preferences
  • Continued theme work

Installing BETA 10

1) Quit or Exit from Postbox so that it is no longer running.

2) Download and install using these download links:

3) If you are using BETA 6 or earlier, or if this is the first time using the BETA, you will be presented with an option to migrate settings from earlier versions of Postbox:


Select "Postbox 3, 4, or 5" then select your desired profile. We recommend using test profiles with test data only.

The migrator will copy the following to a new Postbox data folder:

  • Account Settings
  • Local mail and POP3 mail
  • Signatures, Responses, Quick Post, Topic, and Address Book data
  • Select preferences (not all) from your previous profile

The migrator will not copy:

  • IMAP mail - this will need to be redownloaded
  • Passwords
  • Search indexes - you'll need to reindex by selecting Tools menu > Indexing > Index All Folders
  • Preferences set using the config editor

As we progress through future BETA releases, we'll add more preferences to make migration as seamless as possible.


Postbox is based on Mozilla code, and as of Firefox Quantum, Mozilla has discontinued support for add-ons. As such, a decision has been made to remove add-ons functionality from Postbox at this time.

Config Editor

The Config Editor has been removed from Postbox. Critical preferences that have traditionally been accessed via the Config Editor will be added to our profile migrator and to the Preferences UI over time.


The macOS theme is nearly final and we're ready for your feedback.

The Windows theme is still a work in progress, and we ask that you withhold theme feedback for now. We still have a lot of unfinished theme tasks on Windows.

What to Look For

At this time we're looking to stabilize existing functionality found in the production version of Postbox.

Specifically, if something is not working in the BETA that is working on the production version of Postbox, then we'll want to know about it.


Please let us know about any issues that you find on the Postbox BETA Feedback page. 

(While we appreciate each submission, and we receive thousands, please note that we cannot respond to the comments you submit.)


Thanks very much for helping us test this exciting new release!

- The Postbox Team