Every step below is from an official source
Switch from Outlook.com to Proton Mail
What to do at Outlook.com 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 Outlook.com, before you leave
Turn on forwarding in Outlook settingsBefore
Go to Settings > Mail > Forwarding, turn on Enable forwarding, enter the new address and select Save. Select Keep a copy of forwarded messages to keep the originals in your mailbox.
Turn automatic forwarding on or off in OutlookMicrosoft. ReviewedExport your contacts to a CSV fileBefore
Open the People page, then select Manage contacts > Export contacts. Choose all contacts or one folder. The file saves to your Downloads folder.
Export contacts from Outlook.com or Outlook on the webMicrosoft Support. ReviewedKnow that closing deletes your email and contactsAfter
When you close your Microsoft account, your email and contacts are deleted from Microsoft's servers and can't be recovered.
Close your Outlook.com accountMicrosoft. 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 Outlook.com 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.