Connect iCloud
Mail, calendar and contacts — three read-only connections from one app-specific password, over the open protocols Apple already speaks (IMAP, CalDAV, CardDAV). No app to register, nothing to enable.
1
Generate an app-specific password
appleid.apple.com → Sign-In and Security → App-Specific Passwords → generate one, named "Postern". Apple shows it once.
2
Paste into the Console
Console → Sources → iCloud → your Apple ID + that password → Connect. One credential fans out into the three connections.
iCloudapp-specific password · mail · calendar · contacts
Apple ID
App-specific password
xxxx-xxxx-xxxx-xxxx
Connect iCloud
WHAT STANDS WHEN YOU FINISH
Three read-only connections from one credential — mail envelopes, calendar events, contacts — syncing on their own cadence, answerable by any agent you have granted those sectors. Revoke the app-specific password at Apple and the gate goes quiet; nothing else of yours is touched.