Gmail
OAuth, XOAUTH2, the same sync core.
Dock, don’t migrate.
One sync core over IMAP and SMTP. OAuth only for tokens.
Microsoft 365
Entra ID, XOAUTH2, no Graph silos in the core.
iCloud
App password, IMAP, done.
IONOS
Classic IMAP/SMTP, host in the EU.
Strato
German host, same core.
Your own IMAP
Host, port, user, password. AES-256-GCM at rest.