Every step below is from an official source
Switch from Proton Mail to Outlook.com
What to do at Proton Mail before you leave, what Outlook.com 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 Outlook.com, when you join
Forward your other account's mail to Outlook.comDuring
Outlook.com no longer lets you connect new third-party accounts such as Gmail. To read that mail in Outlook.com, set up forwarding in your other account.
Connected email accounts like Gmail and Yahoo not working in Outlook.comMicrosoft. ReviewedImport contacts from a UTF-8 CSV fileBefore
Microsoft's contact import guide, which applies to Outlook.com, recommends a CSV file with UTF-8 encoding so all text shows correctly.
Import or export contacts in Outlook using a .csv fileMicrosoft. Reviewed
Then: everything else the switch affects
The steps above are what Proton Mail and Outlook.com 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.