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 6th while Postbox is ranked 8th. The most important reason people chose Mailspring is:
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.
Specs
Question Ranking
Pros
Pro Blocks tracking gifs
Has the ability to block tracking gifs commonly used for read receipts.
Pro One Inbox for all accounts
All messages in one place. But you can select to show only messages from each account either.
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 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 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 Snoozing support
You can swipe to archive / snooze messages and specify when you'd like them to resurface in your inbox.
Cons
Con No support forum
Support for paid product has no online forums.
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 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.