When comparing Mailplane 3 vs Mailspring, the Slant community recommends Mailspring for most people. In the question“What are the best e-mail clients for macOS?” Mailspring is ranked 9th while Mailplane 3 is ranked 18th. The most important reason people chose Mailspring is:
Mailspring has great integration with gmail features and tags.
Specs
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Pros
Pro Can work with mutiple Gmail accounts simultaneously
The tabbed interface included in Mailplane 3 lets users work with as many Gmail accounts simultaneously as they want.
Pro Fast and light
Pro Doubles as a calendar
Mailplane 3 combines Google Calendar and Gmail within a single app.
Pro Easy to use
Mailplane 3 looks just like Gmail's web UI and even imports Gmail shortcuts. If Gmail shortcuts aren't ideal for the user, those of Mac can be used instead.
Pro Supports many plugins
Functionality and features can be extended by using plugins.
Pro Great integration with Gmail
Mailspring has great integration with gmail features and tags.
Pro Neat UI
The UI is very well designed and neat.
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 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
Only in the paid version though.
Cons
Con Written for those who drink Google's Kool-Aid
It's a thinly-veiled skin over the browser interface. Why bother?
Con Can freeze very often
Con Gmail only
Mailplane 3 is a Gmail-only client. It does not support other email providers.
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 Slow updates and bug fixes
It takes months to fix some simple bugs. For example, they can't fix bug with notifications on mac OS from April 2019! Upd: they fixed it after 6-7 months
Con Unstable, have to keep fixing passwords
You have to keep "updating the password" because it continuously finds it hard to sync with multiple Outlook accounts.
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 addressbook
Con No way to see messages as plain text
And HTML is only optional.
Con Does not support Microsoft Exchange
Con Very limited user interface
No way to see the messages as a list, no way to rearrange views.
Con No portable windows (.zip) bundle available
Con Does not support POP
Just IMAP.
Con Does not support multi-user installation (Windows)
But instead installs to the user's home directory.
Con UI is sluggish (Windows)
Click and only after a tiny delay (~half second) something happens.