I was researching WebMail providers, and noticed that most WebMail providers recommended in privacy communities are labelled as proprietary by AlternativeTo.

I made a list of WebMail providers, private or not, to see which ones were actually open source:

Proprietary

AOL Mail: Free

Cock.li: Free

CounterMail: Paid

Fastmail: Paid

GMX Mail: Free

Gmail: Free

HEY Email: Paid

Hushmail: Paid

iCloud Mail: Free

Mail.com: Free

Mailbox.org: Paid

Mailfence: Freemium

Outlook.com: Freemium

Posteo: Paid

Rediffmail: Paid

Riseup: Free

Runbox: Paid

Soverin: Paid

StartMail: Paid

Yahoo! Mail: Freemium

Yandex Mail: Freemium

Zoho Mail: Freemium

Open source

Criptext: Free

Disroot: Free

Forward Email: Freemium

Infomaniak kMail: Freemium

Kolab Now: Paid

Lavabit: Paid

~~Mailpile: Free~~

Proton Mail: Freemium

~~Roundcube: Free~~

Skiff/Notion: Freemium

Tuta: Freemium

Unless I'm missing something, it seems like people overlook this when deciding on WebMail providers. Is it a distinction between a proprietary backend server and a proprietary app, or is there a different way to decide if a WebMail provider is proprietary vs. open source? Lavabit was labelled proprietary by AlternativeTo, but open source by Wikipedia.

Note

If I have labelled an open source WebMail provider as proprietary by mistake, please provide evidence by linking to the source code, and I will happily change it.