We'd like to point out that you can connect a custom domain to a Proton Mail account, which would make you eventual migration easier. Additionally, we also offe

We'd like to point out that you can connect a custom domain to a Proton Mail account, which would make you eventual migration easier. Additionally, we also offer a free email export tool: https://proton.me/support/proton-mail-export-tool https://proton.me/support/proton-mail-export-tool. Finally, Proton was created in order to make online privacy available to anyone, so we will always try to maintain interoperability as much as we can.