For everyone not on Google or Microsoft

A calendar and a mailbox should not require picking one of two American vendors. iCloud connects over CalDAV, and any IMAP/SMTP mailbox connects with the credentials your provider already gave you.

iCloud calendar over CalDAV

An app-specific password from Apple is enough. Sell reads and writes the calendar you choose, the same way it does with Google and Microsoft.

Any IMAP and SMTP mailbox

Your own domain, your own host, your own rules. Sell reads for context and sends through SMTP from your address.

Setup

Open Connections in the portal.
For iCloud, generate an app-specific password in your Apple account and paste it.
For IMAP, enter the server, port and credentials your mail host publishes.

Limits

IMAP has no OAuth, so it is a stored credential rather than a revocable grant. Use an app password where your provider offers one.
Some hosts throttle IMAP hard, which shows up as slower mail context rather than an error.