Proton Mail encrypts your mailbox end-to-end, so it doesn't offer IMAP directly. Instead, Proton's Proton Mail Bridge app runs on your computer, decrypts your mail, and exposes a local IMAP/SMTP server that Mailspring connects to. Bridge is available on Proton's paid plans.

Setting it up

1. Install Proton Mail Bridge, sign in to it, and leave it running. Bridge shows a Mailbox configuration panel with a generated password — this is not your Proton password, and it's what you'll give Mailspring.
2. In Mailspring, go to Preferences > Accounts, click +, and choose IMAP / SMTP.
3. Enter your Proton address. Mailspring recognises protonmail.com, protonmail.ch, and pm.me addresses and fills in Bridge's defaults: server 127.0.0.1, IMAP port 1143 and SMTP port 1025, security STARTTLS, and Allow insecure SSL checked (Bridge uses a self-signed certificate). If you use a custom domain with Proton, type these values in yourself; check the ports against what Bridge shows, since they can be changed in Bridge's settings.
4. Paste the Bridge password into both password fields and connect.

Why there's a "Folders" container folder

Proton doesn't allow folders inside folders, but Mailspring needs a place to create its Snoozed folder. For Proton accounts the Custom Container Folder is preset to Folders, which is where Bridge exposes your custom folders, so Snoozed appears alongside them. Leave this as it is unless Bridge tells you otherwise.

Troubleshooting

• Certificate Error (IMAP or SMTP): make sure Allow insecure SSL is checked for both IMAP and SMTP. Bridge's certificate is self-signed.
• Authentication Error: you used your Proton password instead of the Bridge password, or Bridge isn't signed in.
• Messages appear and disappear from the inbox: older versions of Bridge and Mailspring didn't get along; update both. If it persists, select the account in Preferences > Accounts and click Rebuild Cache....