I have a service that I’m providing as a beta at the moment. The code is all Free Software (Debian, Stalwart, Bulwark) and the data is sovereign to Europe (SEAL level 4). Hosted in Sweden, Finland and soon Norway. All run by a Swedish company without any US corporate involvement so you’re not subject to Chinese or American jurisdiction (which means no US Cloud Act) but you are subject to European jurisdiction so you have GDPR and DSA protection, e.g. the right to be forgotten. Beta access is here: https://sverige.email/en/sign-up/

Stalwart Labs have a new tool called “Vandelay” https://github.com/stalwartlabs/vandelay which does import / export via IMAP and JMAP. Pretty slick.