When comparing Claws Mail vs Mailspring, the Slant community recommends Claws Mail for most people. In the question“What are the best native e-mail clients for Linux?” Claws Mail is ranked 1st while Mailspring is ranked 6th. The most important reason people chose Claws Mail is:
Claws Mail is a low resource e-mail client that is often default in low resource Linux distributions.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Low resource client
Claws Mail is a low resource e-mail client that is often default in low resource Linux distributions.
Pro Open-Source
Pro Has many plugins for extra functionality
Pro Traditional user interface
Pro Clean interface with 5 layout variants

Pro Saves emails in uncompressed readable format
Allows one to read archived emails with a text editor.
Pro Has very strong filters
The filters a much stronger than in thunderbird.
Pro Saves passwords in encrypted format
Pro Has many themes
Pro Also supported on Windows
This makes switching the platform easier.
Pro Good support for Google's services
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 Blocks and freezes all the time
Con Saves passwords in plain format
Con Too much bloat
some people dont like the simple design of slypheed so they forked it and created claws mail but it has become so bloated since itsbeginnings.

Con Saves emails in uncompressed format
This takes more space on the HDD.
Con Interface looks old and setup is not easy
The icons as well as the rest of the interface look like an application from the early 90's, with this there is also no easy setup options built in like the more modern e-mail clients.
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.
