Yahoo Mail, like its competitors, can retrieve messages from other accounts, enabling you to manage multiple addresses from one location. The mail provider, however, can't use Internet messaging access protocol, or IMAP, to download data from an incoming mail server -- the service responsible for routing messages to an email client -- and instead relies on post office protocol. POP, unlike IMAP, does not synchronize your messages and folders across mailboxes, so changes you make to your messages in your Yahoo Mail inbox won't apply to the same messages in your other account.
Sign in to Yahoo Mail. Click the gear icon from the toolbar and then select "Mail Options" from the drop-down menu.
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Select "Mail Accounts" from under Advanced Options in the left pane. Click the "Add" button.
Create a name for the alternate account and enter the email address into the appropriate field. Click "Continue."
Type your account credentials into the "Username" and "Password" fields and then enter the address of the incoming mail server into the "POP Server" field ("pop.gmail.com" for Google, for example).
Select the folder to import the messages to using the "Deliver Mail To" drop-down menu.
Select "Use SSL" if the incoming mail server requires secure communication. Check "Use Filters" to employ your personalized filters and "Retrieve New Mail Only" to prevent Yahoo Mail from downloading older messages.
Check "Leave Mail on POP Server" to access your messages both in Yahoo Mail and from your other mailbox.
Click "Save," then "OK." Log in to your other account and open the message Yahoo sent you to verify your email address. A Web page appears confirming the verification of your alternate account.
Click "Continue" on the page to return automatically to the inbox. Click the gear icon, click "Mail Options," and then click "Mail Accounts."
Enter your preferred display name into the "Sending Name" field.
Type the email address you want replies sent to in the "Reply-to Address" field. Click "Save" to finish setting up the alternate account.