For several years I have been a happy user of Postbox, an email client that had some advantages over Thunderbird, even though it was based on it. This email client will be sold by Postbox-inc.
Obviously Postbox-inc who sell the licenses for the Postbox Client now tries to invent the wheel twice and to sell their customers for stupid.
A nice design alone does not make a good email client and a good email client either contains all necessary components or, in case of doubt, I have it extended accordingly.
Add-ons
Postbox is built using Mozilla code, and as of Firefox Quantum, XUL-based 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.
If you believe the statement, then Mozilla, who provide the Thunderbird client (for free), discontinued support for addons.
I simply downloaded the daily (the last (V62.0a1) client to show that this is untruthful. As you can see, add-on are still supported.
So of course I ask myself (apart from the design) what functions remain that justify the purchase of the software?
So let’s take a look at the so-called advantages of postbox:
Email Client Features
| Feature: | Thunderbird: | Postbox: |
| Imap Limiting (limit fetch per folder) | No | Yes |
| Dictonaries | Yes | Yes |
| Add-on | Yes | No |
| Encryption (PGP) | Yes | No |
| Customize design (Themes) | Yes | No |
| Support Windows | Yes | Yes |
| Support OSX | Yes | Yes |
| Support Linux | Yes | No |
| 64bit support | Yes | No |
So in summary, the only thing that remains is a crippled Thunderbird client with no significant advantage. What my personal opinion is after an extensive test of version 6 of Postbox.
So the best advice I can give is, save your money and customize thunderbird by yourself.