When comparing Postbox vs Mailspring, the Slant community recommends Mailspring for most people. In the question“What are the best e-mail clients for Windows?” Mailspring is ranked 4th while Postbox is ranked 6th. The most important reason people chose Mailspring is:
The UI is very well designed and neat.
Specs
Question Ranking
Pros
Pro One Inbox for all accounts
All messages in one place. But you can select to show only messages from each account either.
Pro Blocks tracking gifs
Has the ability to block tracking gifs commonly used for read receipts.
Pro Quick Bar for macros
Quick bar allows you to add a variety of content to your message. It will let you reuse URLS in your Sent folder, and autocompletes against canned responses, signatures, and topics.
Pro Neat UI
The UI is very well designed and neat.
Pro CPU / battery efficient
Mailspring uses a C++ sync engine designed to be as efficient as possible, so you can leave the app running and not see your laptop battery life drain away.
Pro Unified inbox
Using a single inbox for all of your email accounts helps you get more done in less time. Mailspring supports every major mail provider—Gmail, iCloud, Office 365, Outlook.com, Yahoo!, and IMAP/SMTP—so you have a single, streamlined command center for all your messages.
Pro Snoozing support
You can swipe to archive / snooze messages and specify when you'd like them to resurface in your inbox.
Pro Unlimited number of accounts
Pro Read receipts and link tracking
Activity tracking is built into Mailspring so you get notified as soon as contacts read your messages and can follow up appropriately.
Pro Great integration with Gmail
Mailspring has great integration with gmail features and tags.
Cons
Con Way too expensive
Currently the price is at 45€ (US $40). That's ridiculous for an E-Mail client. The design alone is pretty but neither does that or the functionality in any way justify the pricing.
Con No support forum
Support for paid product has no online forums.
Con Can't use without creating a Mailspring ID
There is no need to create a third party ID for an email client. What if the Mailspring closes in the future - can't install a previously downloaded Mailspring software any more to continue using it or access your stored emails?
Con No addressbook
Con No portable windows (.zip) bundle available
Con Pro subscription model
To use some features, like contact profiles and link tracking more than a few times a month, you need to pay for a Pro subscription.
Con No way to see messages as plain text
And HTML is only optional.
Con Very limited user interface
No way to see the messages as a list, no way to rearrange views.
Con UI is sluggish (Windows)
Click and only after a tiny delay (~half second) something happens.
Con Does not support multi-user installation (Windows)
But instead installs to the user's home directory.
Con Does not support POP
Just IMAP.