When comparing Claws Mail vs ProtonMail, the Slant community recommends Claws Mail for most people. In the question“What are the best e-mail clients for Windows?” Claws Mail is ranked 6th while ProtonMail is ranked 10th. 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 Built-in end-to-end encryption
Pro No personal information needed
To create an account you don't need to give any personal information, just choose username, domain, and password. Even the recovery email address is optional.
Pro Privacy respecting
User data is protected by strict privacy laws because all servers are located in Switzerland.
Pro Open Source
Pro Mobile apps
Apps for Android and iOS available.
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 Javascript-intensive Webmail
Free accounts are required to use Javascript webmail and encryption is done in the browser. Javascript cryptography is harmful for security and should not be used.
Con Encryption Keys are stored server-side
The keys are generated during account creation. Using your existing keys is prohibited and ProtonMail must store and control the private keys. Encryption cannot be secure unless the user controls the private keys.
Con Still lacks some useful features
At the moment, there is no calendar feature and contact management is quite poor. They will, however, start to work on it.
Con Ties to US Investors
ProtonMail takes money from US Investors
Con No IMAP and SMTP support for basic accounts
Basic accounts are restricted to the Protonmail web client. Support for IMAP and SMTP is available with a paid subscription only, and as such with basic accounts it is not possible to send and receive email with external applications.
Con Overly expensive for desktop support
Con Requires phone number for login
If you sign up over Tor or a VPN, it will require email or phone number verification. Email verification is disabled if you use an email that isn't from Google or Outlook.
Con Not fully Open Source
