Every step below is from an official source
Switch from Yahoo Mail to Outlook.com
What to do at Yahoo 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 Yahoo Mail, before you leave
Download your Yahoo mail with an IMAP appBefore
Use Yahoo's IMAP settings to download your mail into an app like Thunderbird, Outlook or Mac Mail. Large accounts can take several days or longer.
Download your email from Yahoo Mail with IMAPYahoo. ReviewedChoose to keep mail on the serverDuring
If your email app asks whether to keep mail on the server, select yes. Otherwise your email is deleted from your Yahoo account as it downloads.
Download your email from Yahoo Mail with IMAPYahoo. ReviewedDownload what you need before closingAfter
Closing your Yahoo account removes access to your Yahoo Mail messages and folders, so download anything you want first. In most countries you can reactivate by signing in within 30 days.
Close or reactivate your Yahoo accountYahoo. 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 Yahoo 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.