so hats bei mir letztens beim Rechnerumzug mit Mail manuell funktioniert
These are the files you need to copy over from your user's Library folder:
for Internet Accounts
.../Accounts (folder)
for Mail
.../Mail (folder)
.../Mail Downloads (folder)
.../Containers/com.apple.mail (folder)
.../Preferences/com.apple.accounts.plist (file)
.../Preferences/com.apple.accountsd.plist (file)
.../Preferences/com.apple.mail-shared.plist (file)
.../Preferences/com.apple.MailMigratorService.plist (file)
.../SyncedPreferences/com.apple.mail-com.apple.mail.vipsenders.plist (file)
.../SyncedPreferences/com.apple.mail.plist (file)
for Keychain
.../Keychains (folder)
for Internet Accounts
.../Accounts (folder)
for Mail
.../Mail (folder)
.../Mail Downloads (folder)
.../Containers/com.apple.mail (folder)
.../Preferences/com.apple.accounts.plist (file)
.../Preferences/com.apple.accountsd.plist (file)
.../Preferences/com.apple.mail-shared.plist (file)
.../Preferences/com.apple.MailMigratorService.plist (file)
.../SyncedPreferences/com.apple.mail-com.apple.mail.vipsenders.plist (file)
.../SyncedPreferences/com.apple.mail.plist (file)
for Keychain
.../Keychains (folder)