Every step below is from an official source
Switch from Proton Mail to iCloud Mail
What to do at Proton Mail before you leave, what iCloud 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 Proton Mail, before you leave
Set up forwarding on a paid planBefore
Forwarding needs Proton Mail Plus, Proton Unlimited or another paid Proton Mail plan. In settings, select Forward and auto-reply, then Add forwarding rule. The recipient must confirm before forwarding starts.
How to use email forwardingProton. ReviewedKnow forwarding outside Proton ends end-to-end encryptionDuring
If you forward to a non-Proton address, end-to-end encryption is turned off for all email to and from that forwarding address.
How to use email forwardingProton. ReviewedBack up your mail with the Export ToolBefore
The Proton Mail Export Tool is available to everyone. It saves all your emails and their metadata as EML and JSON files. Large inboxes can take a few hours.
Know your address can't be reused after deletionAfter
After you delete your Proton account, no one can use your username again, including you. A deleted account can't be recovered.
How to delete your Proton AccountProton. Reviewed
At iCloud Mail, when you join
Import contacts from a vCardDuring
At icloud.com/contacts, select the Add button, choose Import Contact, then select a vCard. Imported contacts are added to the All Contacts list.
Import, export, or print contacts on iCloud.comApple. ReviewedImport old mail to a custom domain addressDuring
If you use your own domain with iCloud Mail, you can import existing emails from your previous provider on iCloud.com. This isn't available for all providers, and you need enough iCloud storage.
Move other emails by hand with MailDuring
You can also move emails into iCloud Mail yourself using the Mail app on iPhone, iPad or Mac.
Then: everything else the switch affects
The steps above are what Proton Mail and iCloud 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.