Every step below is from an official source
Switch from iCloud Mail to Proton Mail
What to do at iCloud Mail before you leave, what Proton Mail needs when you join, and everything else the switch touches. Your full plan is built in your browser and stays there.
Official sources reviewed . How we review sources
At iCloud Mail, before you leave
Forward iCloud Mail to your new addressBefore
At icloud.com/mail, open Settings, select Mail Forwarding, tick Forward my email to and enter the new address.
Automatically forward email in Mail on iCloud.comApple. ReviewedDecide whether to keep forwarded copiesDuring
Select Delete messages after forwarding if you don't want copies kept in iCloud. Keeping copies uses your iCloud storage.
Automatically forward email in Mail on iCloud.comApple. ReviewedExport your contacts as a vCardBefore
At icloud.com/contacts, select contacts, or use the Actions button to Select All Contacts. Then select the Share button and choose Export vCard. This needs a tablet or computer.
Import, export, or print contacts on iCloud.comApple. Reviewed
At Proton Mail, when you join
Start Easy Switch in Proton Mail settingsDuring
Sign in at mail.proton.me and go to Settings, All settings, Import via Easy Switch. It imports email from Gmail, Yahoo Mail, Outlook or any IMAP provider. Some providers also allow calendars and contacts.
How to migrate to Proton using Easy SwitchProton. ReviewedCreate a Yahoo app password firstBefore
To import from Yahoo Mail, first generate a Yahoo app password for Easy Switch to use.
How to migrate to Proton using Easy SwitchProton. ReviewedChoose how far back to importDuring
You can import all messages or only the last 12 months, 3 months or month. Imports start with the newest messages, and you can run one transfer at a time from the same account.
How to migrate to Proton using Easy SwitchProton. Reviewed
Then: everything else the switch affects
The steps above are what iCloud Mail and Proton Mail say. Your full checklist adds the accounts, codes, payments and devices that depend on your email provider, in a safe order. It's built from your answers, in your browser.