ProtonMail is the fastest growing, truly secure email service. It also happens to be way to get a custom email domain ($5/month compared to Google’s $30).
The ProtonMail bridge is a free app that allows you to use any email app on your computer – notably Mutt.
Web Interface Hypothetical Vulnerability
The ProtonMail bridge overcomes one relatively obscure vulnerability assuming a very hypothetical situation where the owners were forced to hack their own customers to obtain their private keys, something that is against Swiss and international law.
The bridge prevents this hypothetical risk since only you can change what it does and obtaining an open-source version of the bridge – that has been vetted by security professionals.