Hi I just installed Mailspring on Linux Mint and while it imported Thunderbird without a problem it gives me a problem with Protonmail.
Did you download the proton bridge?
Make sure the security is set to NONE
Proton Support Documentation:
Yes, you must run ProtonBridge. ProtonBridge gives you a setup screen with a generated pw to put in to your IMAP client.
THE PROBLEM is … last time I tested it, Mailspring still does not work correctly with ProtonBridge (emails appear/disappear/appear in the inbox after 5 minutes). If someone knows differently, or it has been fixed, please respond. I may try testing it again, but it had been claimed to have been fixed a few times but the problem persisted, last I knew.
That said, it Thunderbird works correctly with protonbridge, as does Outlook and Postbox
EDIT: I just re-installed Mailspring on macOS and the problem continues for me anyway.